The 919 got dumped in the grass this weekend. (I wsn't there for the event, so I didn't see what was touching the ground) It has frame sliders on it as well.
I found grass on the chain, rear tire, frame sliders, bars, and a little loose stuff on the crankcase cover. Not a scratch on it.
I've been told that frame sliders are (mostly) for slow speed drops, and engine guards are for high speed. We went to a "bike gathering" yesterday, and someone who's been riding much longer than me approved of the pucks on my bike.