Don't get me wrong, I know that there are difficulties in manning and utilizing defected warships against the faction they defected from, but by this point in the war, I'm kind of confused as to why the UEF hasn't at least sufficiently overcome those problems to use those ships in times of great need.
They're certainly desperate enough, and merely having them in reserve for the defense of Earth would be a vast boon to the UEF's lines of defense.
As for the difficulties, well...
1) They've had quite some time to examine and learn the different tech, especially with full, experienced crews on-hand to explain things where needed. And they've certainly had time to train new crews in their operation, in addition to supplies and equipment needed for repairing/restocking/etc via the two Tev logistics ships in their possession.
2) Given how apocalyptic the Tev invasion has become, how dire the UEF's position is, and what will happen to them should the UEF suffer total defeat, I would imagine that some of the defectors would be willing to help crew the ships if need be, at least when it comes to Steele's big push on Earth.
3) Those ships couldn't be used for the Shambhala project (Anemoi-class ship aside), since they are flat-out warships that have already been produced and supplied, and how wasteful cannibalizing them would be.
4) Things are desperate enough that, even if they couldn't be used with proper effectiveness, ANY use you could get out of them would be worth it. And with the threat of total defeat imminent, there's no point in completely holding them back even longer. Keeping them as a surprise asset, sure; I can get that. But not even the Fedayeen seems to think such a thing is possible, let alone likely.