still, by just being good at everthing (but king at nothing) its still unbalanced. what would you attack it with???
Well since it's still a capital ship it's vulnerable to other capital ships and of course, bombers.
Either just sidle your capital ship up to it one-on-one and pound it to bits (taking damage yourself, but winning since it's one-on-one and its not a specialized capital ship) or strip its fighters away with your fighters (or a flak-frigate) and use bombers to neutralize it. Really they aren't that hard to take down individually.
The value of these ships is twofold:
1. They are effective when used en-mass
2. They ease the logistics night mare and micro-managing of forming and maintaining a number of fleets. Instead of building a fleet of anti-fighter, frigate and capital ship units in the most effective ratio, you can take whatever specialized units you have on hand and back them up with these. Hence the term "backbone" of the fleet.
Of course by being "decent" at most fleet functions they are obviously not as powerful as the specialized units. In my opinion this is balanced by the aforementioned logistics problem it solves.
And I do feel it fits in well with the TEC doctrine of strength through numbers. Though not so much the cobbled-together-out-of-merchant-ships theme. Though it does say they are building capital ships specifically for battle so maybe it does fit in.