As Eldomm said--keep at least one head. For the rest, you get experience for reducing the rat threat, but no final treasure. Why should you? Many-As-One is a composite form, so nothing will be hoarded by "it," since "it" is all the rats in the locality. Whatever you'd find, you would find earlier and spread throughout the area.
Ironically enough, the creators of PS:T satirize the whole idea of combat-with-expected-rewards in the Modron Cube's dungeons, with their useless "goodies."
