Doesn't make any sense to me, his value now lies in being 'the dressing room guy' and his experience off the pitch, but whether that's enough to stake another year's wages on is questionable. Makes even less sense because we have Chambers and Holding with +1 years experience, particularly Chambers. It will be a case (again) of the lucky one getting a year out on loan and the other one here wasted doing nothing.
Obviously you can't effectively force Mertesacker out and then sweet talk him into sticking around to start his coaching badges with us (effectively on the coaching staff), but that would've been ideal. There's a balance to be struck between keeping the old heads around and letting the young players come through. Maybe Wenger sees a real lack of leadership throughout the squad and reckons it's worth keeping him around.