How do you prove a bad player to be good? That's silly, mate
No, some are merely trying to convince you that Maguire isn't the worst player in history as some of the less rational folks (to put it mildly) would have you believe. He's actually been playing quite well since Ralf took charge. He's no Baresi by any stretch, just an average PL CB who has improved his performances of late. Nothing more than that.
Some may also feel compelled to defend a guy who has been treated disgracefully this season, not only by the ABUs who love to stick the knife in at every given opportunity but also by his own fanbase. Some of the stick he's received since the Euros ended has been way out of line. Underperforming or otherwise, no one deserves that.