Singing live is what 'live' music is all about. If a band/group isnt up to that challenge, then they have no right to be on a stage or selling records.
In my younger days I roadied for a couple of local bands, flew the PA etc, and well do I know that a singer can be a bit flat. Or a guitarist hit the wrong note/miss notes, but at the end of the gig, if its been a good set then the punters will forgive the band.
But if a bunch of good looking but tallentless waster only mime onstage, I wont be going to see them or buy their tackie pap.