Indeed. But then when the Cahill-Luiz-Azpi trio have established themselves as solidly as they have, I understand why there isn't rotation just for the sake of it. Even though we've had trouble keeping clean sheets lately, it's not got to the stage where it feels sensible to bench the de facto captain in Cahill (but then if it means that Azpi gets to be captain, I am 100% in favour ).
All the same, Ake definitely looks like he should be our '4th choice' centre-back, ready to step up at a moment's notice and perhaps stake his claim from there. I don't think Zouma is comfortable enough playing in a three yet.