This is actually a topic I have some strong feelings about. About a month ago I was cruising down the road at 40mph (I know cause there were cops in the area and I was in "compliance" mode). There are no cars in front of me, maybe some on comming way up there and maybe some trailing me but a ways back there... All of the sudden a deer comes running out from the side of the road - looked to me like a big one - I didn't see him until he was already in the road way; I didn't even have a chance to think about brakes I was so totally not ready (maybe that was my fault) but... I T-Boned the deer at full speed - 40 mph.
I remember hearing all the plastic breaking, I remember feeling a rather sharp jolt at the stopping impact. I don't actually remember going down or how far we slid before comming to a stop - though I don't think we slid much. My helmit shows slight scraping impact. Next thing I remember is being up and groaning because shoulder hurt. I didn't realize something was more seriously wrong until I tried to move my arm and felt something moving around in there, turns out my collar bone was broken (5 or 6 places).
The key for me was a few days later when I went to look at the bike at the impound lot where it had been towed. The frame slider on the right side was ground down maybe a half inch or more. My knee suffered some rather deep scrapes but nothing like I would have had if the slider hadn't been there. As far as I know the frame held it's integrity and it looks like they're going to fix my bike. Based on what I saw those sliders saved me from having some REALLY serious issues w/ my knee, might even have saved my leg. I will always try to get sliders put on any bike I ride after this.
BTW, had to get surgery and now have a metal plate bolted to my collar bone - in another 3 or 4 months I'll get the plate removed and have a reasonably strait and healed collar bone.
I think in the end it depends on the kind of impact you have - which is something you can never predict - strong lateral forces while dragging the slider on the ground will have a strongly negative impact on the engine mounts. I don't think mine slid very far, nevertheless I would rather waste the frame or the slider than my knee.
But of course to each their own - just don't call me a whoose for wearing sliders I know why I have them now. :)
-Dennis