Bill...what happened to the thermal coating response that most of us expected to find in this post?
I could say that they had it because they needed everything they could get and since the M62 was substandard they prettied it up to polish a turd.
That wouldn't be true though would it?
To the best of my knowledge and mostly from information that WillWren provided the reason is to help thermal transfer. The M90 didn't get the coating because of the cost and being that the M90 doesn't spin as fast, there isn't a need to cool any faster with speicalized coating.
I in fact have found that most if not all 96 M90'* were coated.