Voting doesn't have to be just about XP. I think that giving a node a "++" can show your appreciation just as much as a comment. When a node gains a high reputation, then it shows that person that they obviously asked a good question, or had a good idea, or really knows what they are talking about. And giving a good vote can give you the satisfaction of giving that person a "pat on the back" if you will. Sometimes I'll see a node that asks a good question, but I don't know the answer. It seems more appropriate at that time to give a "++" vote, rather than make a comment about something I know nothing about.

*shrug* That's the way I see the voting system anyways.