I doubt Johnny Cash or Jim Reeves flicked through books and said "I'll sing this one" having never practiced it for an audience, stopped to turn a page and half way through, give up and start again with a different song. Or ignore others, reading and sorting whilst some poor bugger is doing their bit. Maybe backstage but not sat next to them...

Like I said, I think the difference is between entertaining and self expression. Do you go mainly to hear others? Then you expect them, regardless of technical ability to make an effort or at least have practiced a song. Do you go to hear yourself? Singarounds are an excellent outlet then, but sing with your eyes closed to avoid disappointment.