Unless U2 does a complete U-turn in singles marketing strategy, I don't see Mercy making a b-side. I always thought it was on the "Complete U2", but I've been told it's not. So, relatively speaking, it's still an unknown song. I think U2 leaked it just enough to see what the reaction would be, and now I think it will be saved for the next album, which could even be titled "Mercy" on the strength of that song (sorry Jick...song is not overrated).
Smile is another song that could be held back for the next album. I don't know if it made the complete U2, but both songs are strong tracks that just need a little touching up.
Like I said, though, if U2 does get interested in singles sales again for this album, then that's the only way I see it being used as a B-side. The hoopla that could surround an "Anniversary" single might cause them to waste one or both of these songs as a b-side. I say "waste" 'cause they deserve to make it onto an album.