What if you lowside your bike and your foot is still under the bike and the footpeg folds up? Seems to me there would be a substantial disadvantage there as the pavement grinds your foot down to a stump. On the other hand there would be a signifigant advantage to solid pegs in that you could just slide your foot out, coast to a stop and jump up virtually unscathed!
As to cylinder guards that have subsatntial protection there is a thread somewhere that shows one with a plastic spool on it but that is the only one I know of. I'll try and find the thread.