And they can wear helmets on those helmets, so their brains are protected while being protected.
Also, RoPC is one of the rarest and most expensive rings (rivaling only invisibility one).
It protects you from the most embarrassing deaths possible - getting transformed into a mushroom/plant while next to an enemy is the next deadliest thing after getting beheaded. Everything else gives you SOME chance at survival (amulet of life saving for example).
Can you polymorph into an intelligent creature and somehow skip the transforming back part to transform into a more powerful one? Like, human->mage->angel?