Where do you get this? People have claimed this before and it makes no sense to me. The song may have less-than-perfect sound quality, but that's because it's a bootlegged copy. The song was removed from the tracklisting at the final stage with Fast Cars. if Fast Cars sounds finished than Mercy probably is too.
That doesn't mean they won't change it (read: fuck it up) before it appears on an album, but it sounds like it's done to me.
Chalk me up with the other people who think it's the best thing they've recorded this decade.