You are beyond grasping the obvious aren't you? Where did I say that defence in structured play was all that counts.
Let me paint some pictures for you - I said the measure of whether a centre is able to cope with his defensive role was whether he was able to contain his man in structured backline play - be it his opposite number or your wonderfully insightful take that introduces the fact that possibly its not always his opposite number that he will be marking - personel aside, I'm talking structured backline play.
The fact that said player misses tackles in broken play is not a good thing, but is NOT a measure of whether he will work out as a centre defensively. No doubt you wont grasp this at all.....but hey who cares.
JP was good at centre and the fact that Plumtree has moved him mean nothing, for Plumtree is an average coach who sees positives in the likes of Joubert, Adi and Bosman at centre