I've always guessed that there is a way to use FGT without losing all of your powers. Even if it's just entering FGT, but then not using Mugetsu and deactivating FGT to save all of the unused energy.
Also, the user understands FGT after accepting their Zanpakuto spirit's blade. So, they can know everything about the attack and how it works without actually having used it and felt the drawbacks of it.
Im trying to figure out why anyone would go into FGT mode and not use Mugetsu? Theirs no evidence to suggest Ichigo can perform any other attacks in that form. The only other tangible thing we've seen him do is flash-step but thats more indicitive of the raise in power from entering the form.
It would go against the very nature of the technique if you were able to switch it on and off, not very "final".