Quartjar played to five people (not counting bartenders) at our last gig. Two of those folks were married to band members, and one was a photographer hired by the venue. The two wild-card attendees loved us!
But I come at this from a very semi-pro position. We make a little $$ every now and then, but mostly not much. That's okay, we're not making a living from it. We do it mostly because we would hate to not do it.
On the whole, I'd love to sit in with any acts that I'm a fan of. Come to think of it, I am usually daydreaming of just that when I'm in the audience.
Well, lunch is almost over. Gotta get back to it.
edit: Also, what Keef said.