Couldn't agree more - I always work on the principle that you NEVER leave a motorised vehicle unattended with the engine running - not even a lawn mower. I've flown many an R22 with the "self-raising" collective and, as you say, frictions can work loose.
If something can go wrong, it almost certainly will sooner or later.
If you are flying the sort of pax who can't be trusted not to shred themselves even after a comprehensive briefing then the only safe way seems to me not to let anyone out until the rotors stop turning.