I like the idea as it's a rather benign way to reward people willing to work with strangers. Not enirely sure how people can abuse it. Either you get a upvote, or you don't. If you get one, awesome. If not, oh well. Personally, I like to see an incentive for folk to give it a little extra.
Not enirely sure I can see how it can be abused. Nobody can be held hostage by folks demanding an upvote AFTER the instance is over. There is NO downvoting which can totally be abused. Obviously you can't vote for friends you queue with since that can be abused just as easily.
No, this system rewards people who cooperate with others when they have no influence over them outside of their performance in the instance. And it harms no one except those who want the rewards yet refuse to play with strangers.
Playing with friends is its own reward.