Is violence ever a good idea?

Hell yes it is. It should be avoided, of course, but it is useful when there are no other means. To use a metaphor, we should use violence like we SHOULD (not do) use brain-altering drugs. If a person is beyond the reach of human help, sometimes psychiatrists need to prescribe drugs to get that person to a place where they can work through their problems through therapy. In the same way, if a situation is beyond peaceful change, I fully advocate brief and judicial use of violence to get a foot in the door, so that the situation may be dealt with fully after a short period of gaining control. So, using a modern parallel, I would advocate a secret assassination of Osama bin Laden, but not war on Iraq. Violence should be used like a surgeon's knife, not a chainsaw.