actually, a fair number of transformations that are used in anime actually have decent reasons as to why they are restricted the way the are. like in dragon ball z, all SSJ transformations drain stamina while they're active, and further transformations have higher requirements to achieve.
in bleach, hollowfication taxes the mind, since the users have to essentially subjugate their inner demons who's only desire is to completely take over the body and consume every living person. so the user either needs an especially strong mind to keep the form in check, or needs to be exposed to enough hollow spirit energy that they get more used to it. this transformation ALSO burns stamina.
actually, thinking about it, most transformations in anime seem to be restricted by how much stamina the user has to be able to maintain it in an actual fight. so, you're statement about transformations is a bit incorrect, they usually DO have decent plot relevant explanations as to why the transformations don't remain active for very long, and they tend to come up with decent explanations as to why later transformations weren't available before hand.
but yeah, i'd rather not have the hero gain some sort of transformation. we already have something pretty close with soul synch when you have baltael's aventail equipped as a regular soulweaver.