Another advantage of having a singaround organiser is you can say to the unready guitar player 'We'll come back to you. In the meantime...'. Or 'Do you want to get your guitar out? We'll be round to you in a minute.'

And the comment about banning melodeons - once you start banning any instrument or kind of song you are on a slippery slope. If a singer is being accompanied inappropriately by others in the singaround it's ok to ask for the culprit to shut up.