The cello for accompaniment by itself may work well in some instances--in fact the viola by itself might well work. But you'd have to work the arrangement out in advance. I'm talking about the the informal session where you really have no idea how the singer is going to do the song. So you can't guess--there may well be a ritard or accelerando where you don't expect them. If however the singer is also playing guitar (which often happens) you can be sensitive to the guitar--and the voice, of course--and tailor the accompaniment accordingly.

I certainly have heard some absolutely wonderful cello accompaniment on record--done, for instance, by Mudcat's Kathy Hickerson on Helen Schneyer's records.

But in an informal session, I wouldn't even try to accompany anybody singing a cappella unless he or she asked for it. As a singer, I know that I often like to be free to sing a song without being locked into anybody else's conception of it--and I know others feel the same.