Though I always build the Kol first (when firepower is at a premium), I consider the Akkan to be an absolutely crucial part of a big fleet. It's usually very easy for the enemy to retreat from any battle they can't win. I can usually only catch up with them for a few seconds at the edge of the gravity well, and the ion bolt ability means I can stop a capship from fleeing and my fleet can kill it.
This was key to my only victory so far, that, and restarting over and over until I had some securable bottlenecks so I could compete for resources without siege-frig rape

. Anyway, once I was ready to go to war I chased down and destroyed all the high-level capital ships of each enemy I took on, thus denying their potent abilities that would've given me a bad day. If I couldn't shut them down with ion bolt that would've been very difficult to do.
It's also an effective colonizer that I don't need to babysit. I quite appreciated I rarely had to track down a colony ship as I swept across the solar system.
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.