And let's not delude ourselves that just because we're going through this shit of him apparently asking for a transfer, that he's suddenly not been one of our most important players (and to be perfectly honest, our most important player for most part) over his time under us.
As for what he offers. Tracks back, can play any midfield/attacking position on the pitch, covers a massive amount of ground and has a great engine (and don't let people fool you into thinking he hasn't), can score a top amount of goals and can lead a team. It's quite amazing that just because of this ridiculous saga and him not having a great season that people have suddenly forgotten how brilliant Rooney actually is.
That's nothing against Mata at all, he's clearly a quality player, but you're kidding yourself if you actually believe that Mata could come in and replicate what Rooney does for the team.