As someone who sings and plays, I would never dream of 'playing along' with a singer unless specifically invited to. I like unaccompanied singing when it's done well. I don't find that unaccompanied singers get a raw deal either; in fact the reverse. I've been in a lot of allegedly 'mixed' sessions where an unaccompanied (and often unaccomplished) singer will drone on for twenty-odd verses, with choruses between each verse, and then the next person, and the next, and the next, will do the same. The songs are almost always done veeeerrrryyy slowly (why????)with lots of ornamentation, which to my ear detracts from what are often fine melodies. After half an evening of this sort of dross, I usually give up and go home. Singarounds are especially prone to this. When the long-song megalomaniacs are in, it's a good night if you get more than one turn to sing or play. It's just plain boring most of the time. If you want an unaccompanied-only session, why not start one? Advertise it well, so I can avoid it.
Tim