I bought a Sledgehammer from Don Purdy last week, and love breaking with it (which was my main reason for it) but I can't get the damned thing jumping right. It's me, I know, because Don picked it up and popped balls all over the place. I'm not the worst player in the joint, and I just can't get my stroke right on those jumpers. Of course, I haven't used it in a match, and won't until I master it and really need it.
As far as whether they hurt or hinder the game, there are one of two situations in the arguments. Either anti-jump-cue-folks have their own ideological vision of what pool should be (in which case there is no debating, since they aren't operating by reason but by taste) or they don't weigh all the tools equally. I think the leather tip is a good example, but a better one might be the mechanical bridge. It could be argued that a good player won't leave themselves a pattern which might require using it... but I'm glad it's there. Shooting with it is hard and takes practice. Just like with a jump cue...