Bottom line is we've got a top-notch catcher already without having to trade for him. The downside to Buster Posey and Joe Mauer is what you'd have to give up to get them. We don't have to give up anything to get Hanigan, he's already here. Consider the whole deal and we'd end up losers if we traded for either of them. I agree with joshua, we've already got the complete catcher (offense, defense, game calling, quiet leader of pitchers, etc.).
Hat off to Ryan Hanigan ... hopefully he's only underrated in the press and not by the brass come contract time.