I actually considered the carrier and the siege-capship to be the useless ones. I may try out the carrier now that its gotten so many endorsements, but it seemed to me it requires too much resources to reach a critical mass where it can decide battles.
The only endorsement I could give the siege capship is that it's much much better at razing planets than other capships are, and very cost-effective compared to siege frigates. I certainly wouldn't choose it to go into battle, but you need some sort of siege ability to properly go to war.
Funny, in my 2nd game (and 1st win) my small-ish fleet consisted of (at one point), one of each but the heart of my fleet from the start was my carrier. I imagine there's a fair bit of difference about which is "best" depending on play style, but also depending on whether you're playing the AI or a human. Humans are crafty devils after all

I'm pretty sure that I went carrier in my 2nd game because I got plastered in my first (in which I'd started with a Kol). It wasn't the capship's fault, but a combination of difficulty with Crystal gathering and utter newness to the game (really wish there had been a tutorial that ran you through the basics of an effective starting strategy).
Anyway, I think the Marza is the least flexible of the TEC caps, but it's siege capacity is good and it's nuke attack seemed to be awfully handy in a brawl as part of a fleet. I imagine it'll be my least used cap, but not entirely unused.