Many monks vote just for their own XP, because they have a chance to gain XP on each voted node. Many of them vote ++ or -- indifferently to content of a node.
While it may be true that monks will vote on their own nodes, how do you know *why* they did it? (ie, 'just for their own XP'?)
I tended to vote on replies to my nodes as a way of saying 'Thank You' to people that helped me out. Since just saying 'Thank you' in a reply tends to get you downvoted, then there's really no other way I can do it.