While I guess there are some singers who can safely be called "bad singers", for the most part, who is a bad singer is very relative. I would rather listen to Bob Dylan, Leonard Cohen, Iris Dement, Garrison Keilor or Karen Dalton than Josh Groben or Celine Dion, but I'm sure there are those who feel just the opposite. Whatever floats your boat.
It's Garrison's show and if he wants to sing on it, that's his choice. If he alienates some of his audience, it's their choice not to listen.
This reminds me of clips on Youtube from Pete Seeger's old Rainbow Quest show. People who don't understand the concept of the show make comments like, "I wish the dude with the banjo would just shut up."

GUEST,sorefingers has obviously spent a good deal of time listening to Prairie Home Companion. A bit of a masochist perhaps?