hmmm actually the akira and defiant where the federations attempt to deal with dangerous matters. While their earlier ships delt with exploration and discovery. the Borg showed the federation they needed muscle. The defiant was made to tackle the borg and the akira was made to help the defiant as well as tackle the dominion, plus sector threats of the future. the akira is also a cheaper ship to make then a galaxy, sovey and nebula. In fact the akira was design for a multi purpose role. this is not my opinion this a direct quote from the creator of the akira peeps

" Alex Jaeger commented on the design in an interview with Star Trek Magazine: "This was my gunship/battlecruiser/aircraft carrier. It has 15 torpedo launchers and two shuttlebays - one in front, with three doors, and one in the back. I really got into it with this one, with the whole idea that the front bay would be the launching bay, and then to return they'd come into the back, because they'd be protected by the rest of the ship."

so there is your akira purpose and design parameters. guess cryptic forgot to follow suit or just felt an escort was easier then a gunship. I do agree having only three styles of ships has made the game a bit dull and not practical in star trek lore.