Yes, it encourages negative behaviour. That said, I think it depends what they are up/down voting.
For example, if it’s a vote on an opinion of another comment, it can be used as a tool for deliberate negativity, but if it’s a vote on something more tangible (say product feedback) it can be a good way to gauge feedback (for example, how HotUKDeals weave community and products smartly together).
Either way I’d advise to be aware of the inevitable claims of bullying. Too many people down-voting a comment runs the risk that the recipient feels intimidated and will react accordingly.