Peaceful protest is infinitely preferable to violent protest. But, protesting any serious issue always carries the risk of violence, from Samuel Adams to Abbie Hoffman to Rosa Parks. The violence may come from the protesters (which, IMHO, makes the protesters look like thugs, e.g. the WTO riots) or the ones protested against (vice-versa, e.g. Kent State).

In my more optimistic moments, I think the shortage of protests in which the threat of violence is a real possibility shows growth as a society.