This is about the tenth time I see this idea in less than two months...

Anyway I am not in favor of these kind of moving turrets (which in they actually are in their essence). This because it would render conventional turrets useless compared to their new relocatable brethren and would also make defenders nearly impossible to push over.

For the vanilla game, I agree...but for modded versions this type of ship could be absolutely key to making certain things viable....

As an example, consider mods meant for extremely large/open maps (such as Aplos' Stars mod)...when you are dealing with dozens of planets that connect to every other planet, being able to rally a small defense quickly would be essential to fighting off small raids...however, these ships would hardly do much of anything against a determined attack...

I do not think having these ships cost tactical slots would be the best approach...I'd rather have a separate limit on how many of these things could be deployed in a gravity well that is independent of everything else...

Barring radical changes made by the devs, there are only three possibilities that I see viable for implementing these:

1. Frigates with disabled phase jump - an ability (either on the planet itself, a structure, or an SB) would deploy one at a time...there are ways to get this so that only so many can be in a gravity well at once, but perfecting it is difficult and I found memory leaks to be common...furthermore, it is AFAIK impossible to get the AI to use this properly, as they will always try to jump these ships and join them in fleets (the AI doesn't "recognize" they can't phase jump)
2. Strikecraft - this is the easiest way to implement such ships, but it has its drawbacks...first, implementing abilities for SC is a little more problematic (though still doable)...probably the biggest issue with this approach is that such ships would be immune to abilities that target frigates but vulnerable to abilities that target SC
3. Mines - I am in the process of trying this out, but since I'm working with a mod that removes mines completely anyway the nice thing about this approach is that (unlike the frigate approach) limiting the number per gravity well is very easy...additionally, these ships are treated different from frigates and SC as far as ability targetting is concerned...the biggest problem with this approach is that you have very little control over these ships, and everything (such as "firing" weapons) has to be done through abilities...

Fantastic idea.  Super +10.  Or make planetary anti-capital ship weapons (like in Stargate) that only purge type vessels are safe from and can destroy!

I don't see how this could seriously harm the def fence dynamic. You make them weaker in both weapons and armor than the fixed defenses, and limit the number you can build (like space mines but also using tac points) so you don't have another corvette spam problem. It would certainly help with the "pirates attacked a planet really far from the front lines, and though fixed defenses are protecting facilities and the planet, nothing can stop those bastards killing my trade ships at the edge of the grav well" problem (and dont say strike craft, because pirate cutthroats eat them)

Im also a fan of the planetary weapon idea that only works on cap ships. If fact it would be great if they specificaly did extra damage to titans, so you would have to destroy infrastructure or your titan would be in danger being too close to the planet (maybe have it stop firing when pop gets under 10 for this planets held by a starbase....or dont)

How about an orbital defense platform that tracks a circle on the inside of the gravity well. It would basically be the same as/similar to a turret, but would rotate around the planet at a fixed speed, making it harder to simply attack a planet from an unprotected side.

 I think the defense need a bit more variation atm. This would balance by being different to deal with, in that it is harder to hit, but also harder for it to hit you. 

It also opens some interesting tactics: For instance, putting three of them on rotation, with one part of the obit passing near a repair platform and your main defensive cluster. You could limit the number in orbit and balance their speed and effectiveness of course...

What about the idea of an upgraded hanger that cost more tac slots, would field the same combined number of fighters / bombers but would also deploy a set number of "system patrol frigates" that had a high level of damage output but a low armor rating, or the opposite of that, low damage high armor.

Would it be possible to due something like this as starbase upgrades?  I'd like to order some baby Orkies that can slow down fleeing enemies.

Not just DS.  Quite a few mods I've seen have it.  In one I'm working on for instance, I've added them.

Since when have the conventional turrets been useful.......lol

They are actually really good early game for protecting factories...sometimes they can be used to protect asteroids from being bombed out as well...

Additionally, they are extremely useful for expanding in the early game when you have to deal with planets that have large militias...

I think the perfect fix for corvettes early on would have  been to confine them to within a system only and limit their numbers just like mine counts.

The biggest issue I hear on a non-jumping ship is the AI tries to jump them anyway with a fleet and they just park on the gravity well edge?

Put a few upgraded Advent turrets togethers and those things damage output becomes insane. Of course having 5 turrets together mean they cover less of the gravity well, and are still vulnerable to bombers.