I seem to be getting a bit of flak here about joining in with the unaccompanied singer.

Let me refer back. I said you need to get the feel for if you can or not, and I said that if you join in with one you have to be good enough to follow.

Subject to that I do tend to feel that unaccompanied singer who want to prohibit anyone joining in may be being a little bit overprecious. If I were to join in vocally on a harmony on a chorus or refrain and get hushed I think I'd feel a bit put out. Why is an instrument so different?