This is the part that baffles me. I cannot, for the life of me, imagine a situation where I'd show up on set and assert that there's no need for the clapper (MOS notwithstanding). There are dozens of practical reasons to use it even with single-system sound, at least in my mind. Maybe I'm overly-cautious, but it's saved my a** before. I even keep a dumb slate tucked in my audio kit at all times, just in case.
I was an AC in a small film once where there was no clapper. I came in late, and so on the first shoot with sound I see no clapper. After some time we go up to the sound guy (since, as is typical, sound is his problem...) and ask why there's no clapper. Long story short, after that point the classic hand clapping in front of camera was put into full action.