Sometimes in a good singaround, when it gets to someone's turn, and they start opening up a case and tuning a guitar, it really does break the rhythm that can build up around the room. And when the accompaniment drown out the words of a song you'd like to hear, that's annoying too. That can happen even with a really skilfully played accompaniment. In fact that can be even more irritating, because the words get in the way of the guitar, and you lose out that way too.

This is a special kind of situation there - I'm not putting down the idea of singing with accompaniment, still less the art of fine guitar accompaniments.
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One thing I've noticed - in a session which is predominantly tunes, when someone sings a song, it often seems to work much better when it's unaccompanied, for the contrast and the concentration.