I agree too. If I'm paying to see them, they better be singing live.
I give a pass for the Superbowl too, although, I would be disappointed if Paul McCartney or Tom Petty, or any rock star lip synced. Even if the sound is terrible.
I guess we are talking about two different types of entertainment acts here. Those that feel they must dance around and put on a big production and those who entertain by "just" singing and playing their instruments. I prefer the latter over all.