Yeah, he's made mistakes and he's had bad games, most of them coming early on in the season whilst adapting but it's as if people expect players to be perfect at all times which simply doesn't happen.
I'd have liked another CB as I don't trust Djourou and Squillaci isn't even really a defender but if you simply focus on errors instead of considering the effect of different partnerships and personalities on the synergy and efficiency of a defence then you're gonna ignore a large part of putting a team together.
From where I'm standing we have two senior defenders who can organise things and two junior defenders who need one of the others there due to their own lack of organisation in Kos' case and inability to concentrate in DJ's case, I reckon Cahill would fall somewhere between those two so would have been more useful than Djourou. It's not perfect but it's a damn sight better than last year.
As for what Wenger thinks, who the fuck knows? One thing I will say though is if Squillaci gets any gametime this season I will consider this summer a complete and utter failure.