Negative.  They are indeed the mainstay (as in any real Naval fleet) of your firepower with the bulk of the rest of the fleet supporting HER.  Generally today we would compare it to the modern nuclear US Navy carrier battle group.  I am playing a med-sized game atm,  It is nearing the end with only myself (TEC), my Vasari opponent (the 3rd and 4th having already fallen under my sword), and a few battered groups of pirates with no home world anymore.

I have 6 Battle Fleets and they are all centered around a battleship of one sort or another.  They also contain 4-20 battle cruisers, bunches of LRM Frigates and various support craft.  I can usually tag team 3 battle groups in and out of a fight and it rapidly wears down even the most hardened enemies.  The key is knowing when do withdraw and when to reinforce and timing the jump to keep a fresh fleet on the firing line.  Devastating.

Just my 2 cents. 

Entrenchment has quick start. You start with 2 scouts. Facilities on the metal/crystal nodes and the cap factory.

I usually only play Vasari, sp here's what I do.

First thing I like to do, is to build the cap ship, send out your scouts then place out the military/civil lab, depending if you're going mil/eco, big map, little map.

I prefer atleast 1 military lab, get my assailents out and tell them to join my egg.
and try to take over and fortify chokepoints.

Then it is about getting enough civil labs to get some mining refineries and in the end the tradeport.
Remember that refineries takes the planet its built on and the planets connected via tradelanes to it.
See another post regarding refineries to see how that work.

But as Vasari. You will use some time to get your economy up, atleast compared to the TEC's ^^

Then one could build 400 assailents, 20 carriers. and watch the despair as you decend upon 'em!
*insert evil laughter*

I beg to differ in the simple fact that more capital ships makes each one less of a focus fire target, especially later in the game with fleets so large they can kill a cap ship super quick.

As Vasari and TEC I like to get more support abilities from my 2nd 3rd and 4th cap ship. Skiratra Carriers and Dunovs are great second and third cap ships for theri healing capacities. Someone once attacked my starbase with a fleet and 3 skirantras. I killed maybe 5-10 ships out of his entire fleet. Missile barrage can kill most frigates and support cruisers, but 2 can kill HCs and carriers. Akkans, Marauders, Revelations and Radiance are all good cap ships to counter MB, which can SAVE YOUR WHOLE FLEET, so its worth considering building a second cap if your opponent is close to or has reached lvl 6 with a Marza. You gotta know when its going to benefit you to get more caps. I usually start considering it after researching the 3rd fleet capacity upgrade.

How about when fighting carrier spams? A couple of Halcyons or Kols can easily turn the game in your favor.

I used to start with the vasari carrier cap ship since I liked using it's heal to keep my frigates alive early.  I didn't know how good the egg was at the time.  I also later found out you can send your cap in first so it gets targeted and have your frigates follow.  I still build the carrier later.  I am sure it is the reason I have low casualties vs AI.

I also like the antorak marauder.  Giving your fleet extra speed helps out more then you might think.  Getting across that well takes much less time.  Either for chasing or running away.  That is really all I use it for.  If I am microing I might put it's phase out on a few targets but if I leave it on auto then he doesn't have enough antimatter left over to use speed.

Here is my personal start:

1) Build a cap ship factory, crystal extractor, 2 metal extractors, 2 impirial labs (I play as Vasari) in that exact order.
2) Build first cap ship then only 1 scout. The first cap ship I choose is either the Egg or Carrier. For some reason, if the carrier is my first ship, I never lose. It may get destroyed, but I don't lose the game if it was built first. My 4-leaf clover so to speak.
3) Select scout as soon as it is built, set explore to auto and activate it.
4) By the time you know the location of the first asteroid, send your capital ship and clear out the militia. Build a colony frigate when you send your cap ship if your cap can't colonise. When the first asteroid is taken, IMMEDIATELY purchase its population upgrade and logistics.
5) By this time your opponents will have started to amasstheir fleet, so build a war lab, research the Kanrack Assailant, and build 6 of them and 4 skirmishers. Now research the available phase missile upgrades.
6) Bring all of your ships into one fleet, with your cap as the flagship and loose fleet cohesion and simultanious jump. Send this to the nearest planet you do not control and research Ice/Volcano colonies, whichever applies.
7) Take the planet you just jumped to after you clear it of enemies. If it belonged to another human player, then move on and attack his/her nearest planet. The reason I recomend this EXACT starting fleet composition is that, when you enter combat, enemy ships will target your light frigates before anything else, leaving you Kanracks free to wreak havoc. If an enemy cap is present, Attack it in unison untill it retreats, then destroy its escort.
8) By now you should have 3-4 planets,at which point you should research your culture structure and trade port. build a trade port on all planets, but a culture center on the "middle" planet. Don't bother with refineries untill later. Now you should be ready to get on your way with your more complex mid-to-late-game tactics.
9) Good Luck

I solely play Entrenchment on quickstart as Advent now so I'll give my personal start up (since everyone's doing it) against as many unfair AI as you want. I generally have every setting on fast, teams are locked on free for all and pirates are inactive.

1) Set the free scouts to auto-explore and set them off.

2) Queue up two more scouts and five disciples.

3) Progenitor Mothership.

4) Build two hostility temples (military labs) and upgrade the logistic slots on the homeworld.

5) Send the cap ship to the nearest asteroid and destroy the siege frigate, colonise the asteroid then kill the LF and send the Progen and disciples to the nearest ice/volcanic world.

6) Upgrade infrastructure, build the extractors and another temple of hostility.

7) Research illuminators. Queue up as many of them as you can while waiting for the lab/research to finish.

8) At around 5-10 illuminators upgrade the logistics on the asteroid and build two temples of harmony. Start researching the first tier cash upgrades and the necessary colonisation tech.

9) If you finish off the neutrals on the vol/ice world before the research is done send them on to the next planet and build a colonising ship. Yeah it sucks compared to the Mothership but it means you can get a head start on the next planet (preferably in the direction of your chosen enemy - I generally pick Vasari as the first since they're harder to fight once all their phase missile upgrades come into effect and I have terrible luck against their starbases >_>) as well as build up your exp quicker. Also remember to pick up any neutral mines you can find inside

10) Colonise the planet and set it up. You should have a fleet of at least 20 disciples and about 10-15 illuminators backing you up.

11) Now for the first attack: Scout out a relatively undefended planet (preferably an ice or volcanic one) and attack. Don't concentrate on any of their fleet too much at first, just aim to take out their extractors (metal is the priority for Vasari, crystal for Advent, not sure for TEC), hit their constructors too if you get the chance. If they have a frigate factory try to hit that with your mothership. It will draw the fire of the assailants/whatever the most and has shield regen to keep itself alive longer.

12) While this has been happening your resources have been steadily accumulating. Research carriers now and queue up at least one at a nearby planet. Your enemy should have its main fleet at the planet by now and it'll likely be full of LRMs. Use your progen to its fullest, as well as your illuminators and get them stuck in amongst all the enemy units to maximise the damage. Disciples should be aiming for any support ships that may have been researched or failing that the cap ship itself. The aim here is not to kill the cap ship first but to get your progen as many levels as possible while you wait for your main carrier force to be built (abilities I go for first are colonsation and shield regen with the third ability point going to either one depending on how the battle's going).

13a) At this point you have a choice. If the battle is going well and you've brought your carriers in (2-1 fighters/bombers, fighters hitting the LRMs and bombers the cap ship) save up a bit of cash and go for a second cap ship with a small supporting fleet. I generally go for the Radiance before the others since it works well in synergy with both the Progenitor and the Rapture (with vengeance). Send that off in another direction to take some more planets/defend against other hostiles.

13b) If the battle is not going well, research the starbase constructor and get set up on a border world around enemy space. Place repair bays and retreat your fleet to them. Start building up your fleet again and get ready for another push. Starfish may be a useful for taking out defences quickly, though not so much for the starbase itself (use bombers for that).

14) You have now successfully made it to the mid-game. It's all up to you now. Suggestions for next few techs would be for whichever resources you are lacking in (depends on what type of worlds you find nearby and how many mines you can build there) and guardians.

Note: This generally only works if you don't get immediately double teamed by rampaging Vasari, since you don't really have the cash to afford such a big fleet and the research for carriers while making last ditch defence attempts. All in all I've had this work two thirds of the times I've used it, and it works especially well if you're facing Vasari straight away since they immediately go for LRMs, which are pretty expensive in the early-game.

Also, yeah I leave building guardians until the mid-game in favour of getting a strong carrier force up and running ASAP. Guardians may keep your frigates alive a bit longer but a 2-1 mix of fighters/bombers will counter the major threats you have this early in the game (LRMs and Cap ships specifically).

So is it any good?

This is almost EXACTLY what people try online against me, and I only win by luck. good plan