It depends on the crash. I've seen quite a few high speed turn one crashes at the creek that damage has been greatly reduced by crash knobs. I had one myself at over 200kph, the crash ground away and the only other damage was to the engine case.
Lowside on a smooth surface and they are great, as are race stand pick ups on the swingarm. In a high side, they won't do much and may point load the impact onto your frame. There is a belief that on rough surfaces they can dig and flip the bike, I haven't seen that though...
In my experience at the track, seeing lots of crashes and being involved in a couple myself

, I'd definately recommend them.
I'm sure Damo wished he a set on when he had the big off.... in his garage..

Dave
2010 Z1000
ex bikes
05 ZX-10R Race Bike - No.77
95 ZXR750R M Race Bike - No. 75
98 ZX9R Race Bike - No. 000
zx6r, zx7r, GPX750, GPX500, lots of KX's.

I ride way too fast to worry about cholesterol