I think most people would agree that it'd take a damn good team for people who prefer working alone to willingly join up and stick with a group. Especially if people in that group are accustomed to doing things in different ways. Also partly why me and Dean prefer to work alone. Some people really wouldn't appreciate the way we go about things. Not morally acceptable to them, you know?
Funnily enough, outside of the powers they've got, some monsters aren't too different from people. Really screwed up in the head types. Then again, you think of guys like Alistair and it's hard to imagine anyone - human or monster - being much like him.