Engine bars also are useful for protecting the jugs from the forces applying torque to a cylinder in a low-speed slide. For high speed slides, bars and protective covers are useless — plan on a write-off and a new bike.
Everyone has personal experience which colors their choices. Mine: I once rebuilt two R1150RTPs into one bike. One was a crashed RTP that had gone down and dragged along on a cylinder for a short distance. This resulted in the front valve cover bolt being stripped out of the head (allowing the cover being lifted off its gasket during the slide), and the possibly momentary unseating the front side of the cylinder base from the block (a bit of oil wetness there, but I do not know whether it was already there or the result of the cylinder flexing rearward during the slide). I believe this was a highly unusual occurrence, as it occurred during a major hit (texting 16 yo rear-ended the stopped bike at 50 mph.). Nonetheless, I have become more conservative when I think of protection.
That said, I believe that tor zero-speed drops, the valve cover protectors are perfectly good. In fact, I put a set of the BMW cover protectors and they saved the right valve cover on my 1250 when I stalled at a stop sign during my first-ever ride on the bike. 
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For my own peace of mind I have moved on to the factory bars, but I completely understand others not liking the aesthetics of crash bars.
