I've said something on this a few months ago in a thread in the Monstrology section:
"As for polymorphing into enemies during combat, I'm not sure. The only use I'd see for it is gaining a ton of Health, but at the cost of worse stats in many departments and who knows how messed up your Polymorph's Deck is going to be."
The only practical uses I could see with it are becoming a creature from a higher-up world for stuff like Critical/Health/Damage maybe, or to polymorph into a boss with lots of Health right before the enemies unload a lot of spells onto you as a means of survival. It could make up for a mostly nonexistent Resist stat (in most cases), but outside of that, the bosses' stats are generally worse. Plus, given their behavior pattern, I'd presume that their Deck would not be built well at all. They wouldn't have very good means to stack blades and healing polymorphs would have a lot of attack cards cluttering their decks (not to mention their healing bonuses are usually pretty bad).
I'd like to see them be available as transformations, however. There's quite a few that would be fun to turn into.
