If I'm singing and I'd sooner be unaccompanied, I just say that's how I want to do it. The same if I'm playing my own accompaniment, and don't want anyone joining in - though sometime there you have to be a bit more forceful.

Generally I prefer a set-up where people play together when they feel like it, unless they've been asked not to. I don't think anyone ever feels their nose put out of joint if there's a request to people generally not to join in - it's trickier when there's one particular person you want to shut up, or if you've started and they've started.

But unaccompanied singing should never be disparaged as a second-best type of singing. As for "It could possibly be argued that to sing solo a capella at informal gatherings is a prime example of placing the musician before the music and the ultimate thrill for the 'control freak." Well it could possibly be argued, all kinds of strange things can be possibly argued - but in most cases I would hope that the argument would fall flat on its face.