I agree... for me, the joy of singing is a physical and emotional pleasure, in and of itself (singing in harmony even moreso). Even if I knew no-one else would ever hear me do it again, I wouldn't stop singing. But singing for other people who appreciate & enjoy it is even more satisfying -- no doubt about it. Best of all, though, is singing with other people...

As an aside -- amongst a group of folkies I know (some of them Mudcatters, as it happens), there is one fellow (named Bob) who does not sing himself (aside from perhaps joining in on choruses). Everyone else likes to get up and perform -- now & then, if not regularly. Since there are several 'Bobs' in this crowd, we have to differentiate between them, so there's 'Quiet Bob', 'Piper Bob', and 'Bob the Audience' -- and I have to say that 'Bob the Audience' may well be more appreciated than any of the rest of us!