That same guy in that thread also states that Mata is comfortably better than Silva...
Anyway... Yeah, the only reason to sell him would be for the money, which, frankly, the club shouldn't need. Great impact sub, even if he's not a first team player week in-week out, which I don't think he should be, especially in a 433.
He's just not got 'it' in my opinion, and there's no escaping his limitations (I don't believe signing pace will actually solve anything), but still he has some valuable assets.
His stats. at Chelsea are good, and Chelsea were quite crap. but I still think that's partly why he got the goals and assists he did. You'd think - well if he can manage that at a poor Chelsea, imagine what he'd do at a much better Man City (say 2011-12.) But swap him for Silva and City are quite a bit weaker as a whole unit, in my opinion. That control they have (or had) over games and that ability to keep the ball deep in the opponent's half is reduced significantly.
I guess it comes down to whether you think his added goals would make up for it.