Yeah but you can trounce on planets with a colony ship also. Engage enemy ships and pull them off to one side of the gravity well (or just on one side of the phase lane) then move in a colony ship from the other side of the lane and colonize the planet right away, then pull it out. Even if the colony ship gets shot up, you only really need to do this twice, after the first two colonies a few seconds of planet capture won't make any difference. Also, wherever you go, you always have a colony ship one phase jump behind anyways, ready to phase in and colonize.
Not particularly convenient, though, seeing as a colony ship then has to recharge its antimatter before it can colonise the planet and has no way to defend itself from harm - if and you don't really want to be faffing about and losing them to enemy fire, because that's 450 credits a go - lose, say, five in that way (quite possible if you're going for a quick capture), and you're not that far off the cost of simply building an Akkan, which comes with fighter / bomber support, a colonise ability that can also net you mines, Ion Bolt, a Targetting Uplink and that occassionally useful power of Armastice.
And what do you lose? You get a cap ship with average dps, bad (no?) combat abilities and that's about it. I can never justify building an Akkan cap ship.
Akkans are great. Their DPS is pretty high and they have quite a few slots for strikecraft. What really makes them is their Ion Bolt power, which is absolutely superb for taking out enemy cap ships on the run - never underestimate 5 seconds in which the enemy cannot move, shoot, or fix itself up. That's a couple of volleys at least, and if your enemy is already weak, that's easily enough to polish them off with a Kol's beams and then a Gauss shot, as well as any frigates you have who can chip in.
That they also come with the Targetting Uplink, which gives your LRMS assured first-strike ability at high levels, and makes flak frigates into strikecraft killing machines at longer ranges makes them absolutely invaluable.
Try it in a game tonight - an Akkan and two Kols together. By the time these ships are level 3 (not like it's much of an effort to get them there), you'll have 4 fighter or bomber wings, a couple of points in Ion Bolt and a couple in Gauss Cannon if you've any sense, and with a few frigates around, enemy cap ships are going to be in serious trouble, as they're unable to move, and get generally plastered by your own caps.