From a military standpoint and canon stand point the Feds should have carriers. But do they really need them in game? Not really. Having carriers only on the KDF side gives them a little special love and incentive to grind up. This last summer, one of my fleet mates got a carrier and that inspired a few more of us to get one so we could have a carrier squad. For me, that was the first time I've actually advanced my KDF toon who had sat around since launch.
For a compromise, why not let Feds have their shuttles do something? Let them launch a single shuttle to be a harrassing force. Been done before in a trek game, Star Fleet Command series and worked rather well. Ranging from suicide attacks to just defending the mothership, the shuttle's would then actually get used instead of being tiny space pets no one sees.
In war, attrition units are needed to provide power projection at a minimal resource and human cost. In WW2 these were PT boats or E boats. Recent examples include the Littoral Combat Ships for the US.
Star Fleet Command was developed from an old board game called Star Fleet Battles (SFB), also based on Star Trek. The Federation was the last of the major races to implement carriers, and to quote SFB: "They set out to build the best carriers in space and succeeded". The fleet carriers could hold large numbers of fighters, but held a relatively weak armament that required them to have dedicated escorts.
This led to other races developing 'Pseudo-fighter's' (PF's) that were essentially stronger attrition units that could take out one or two fighters easily. The Federation countered the PF's with heavy fighters.
Eventually, Space Control Ships were developed that carried a mix of fighters, heavy fighters or PF's, along with PF carriers that carried half a dozen PF's only.
I could see STO following the development of fleet carriers in this manner, although the Klingon carriers with the Birds of Prey are much like the PF Carrier concept already.
Even so, the Federation consists of 150+ worlds that each bring their own experiences with warfare with them and hence to Starfleet. So we terrans, or rather a small fraction of nations on earth, use them.
The thought that because we do so now must mean everyone in the future Federation thinks they must be a good idea is somewhat terracentristic.
The Federation is not just humans in space Starfleet is not just a copy of terran military.
That's not the implication or parallel that I'm trying to draw though. I'm only saying that carriers aren't anything "new" or "special." Not to any race, imo.
Why must carriers be offensive/military only? What confuses me is the argument that the Federation doesn't build war-only specific ships. Why is a carrier war only? I recall in The Next Generation of there being colony ships that held large number of shuttles, more than the Enterprise-D that could help evacuate a planet where transporters were not available.
I honestly think the Federation would have carriers that are designed to do multi-role assignments. In time of war, they can be fited with fighters and support craft while in time of exploration they could be fitted with shuttles and mass amounts of probes. So I do not see an issue with the Feds having a carrier.
Also, the Klingons are the ones now with unique ship classes compared to the Federation. The Feds get Escorts, Science Vessels and Cruisers. The Klingons until recently did not have a Science Vessel but had a Carrier and a Raider class along with their escort and cruiser classes. Now Klingons do in fact have a Science Vessels, removing that from being unique to the Federation while maintaining their unique Raider and Carrier Classes.
I personally think the two factions should have some unique ship classes from each other, tailored to their core believes.The Raider naturally fits the KDF rather well. Carriers arguably should be on both sides. The question is, what should the Federation have that fits them that would be useful against the KDF that fits their style? I could see medical science being useful for the Feds if Crew played a more vital role in ship combat since it was established back in TNG that Klingon Medician was no where as advanced as Federation Medician.
Adding a Federation carrier has been a repeated request, inevitably accompanied by advocates against the prospect.
We currently have no active plans to do this. That being said, it is a topic that has been discussed internally. There are, however, several serious considerations:
First, the Federation has tended to avoid building starships that are explicitly warships. It's true, they've been at war for a long time, and there are in fact episodes with small ships involved in battle, but it's clear there is a bias against this in Starfleet's design mentality. There are non-Starfleet Federation species that might provide a compromise on this point, however.
Second, carriers are one of the few things that the KDF has that distinguish it versus the Federation, the prevalence of cloaking technology of course being the other. We have no desire for the KDF to lose this distinction, but a single-hanger flight-deck cruiser is something that might (emphasis on "might") be considered internally at some point in the future as a way of providing some of the gameplay to Federation players without significantly diminishing the KDF distinctiveness in this area.