Well, competitive multiplayer is a goal for FotG (whether or not the state of the FSO netcode lets it get there is something else entirely), and in that situation under/overpowered ships are bad, at least for something like straight up 2v2 dogfight missions (which as I understand it were the staple of competitive XvT play, not any of the scripted missions). And if you're going to exclude the ships from those missions because they're unbalanced, then why expend the effort on making them anyway?
Also, superfighters don't fit in with our "vision" of how the Imperial starfleet works, i.e. due to politics, paranoia, etc, they are focused on powerful capships & lightweight fighters, in specific contrast to the Alliance. And, speaking for myself, they don't fit in with the feel of the OT, which is good enough reason by itself to exclude them from our TC
Anyway, this is a moot point anyway until someone actually creates the relevant models to a high standard, maybe if someone actually went & did that we could have another discussion about the relevant merits of adding them for use by mission designers, if not by the FotG team themselves. Or someone could simply make a mod to add them after release, it's not like we're going to hunt you down to maintain the purity of our vision