I'm not sure about opposing bigs having good games against him, but I do know he hurts us defensively and offensively. His help D is on par with Bargnani's and he has a terrible touch around the basket. The only time we should be playing him is against really strong/slow bigs but there aren't too many of them around anymore. Last night you could immediately see the defensive intensity pick up as soon as Gray went to the bench, he is just to big and slow.
I agree with your assessment. I shoulds say that I've enjoy his man to man defense at times. Even though Lopez had a strong game against us, he forced Lopez into a couple tough misses early. I can see where the asset of his size and post defense comes into play. It is a shame he struggles in so many other areas.