Unless I did not discover such areas, it seems that the transformation mechanism is only used to progress through the story so far.
It would be nice to add a couple optional areas (doesn't have to be big, just an island with some coins for example) that can be accessed via transformation (e.g. one via flying and one via minimizing) to get some bonus coins, and/or an easter egg (could be a skeleton or a ghost urging you not to battle Satan, or a Godot avatar :P).