It seems people always say how Nadal can do no damage to Djoker's BH side, in contrast to Federer. This is simply an exaggeration. It was clear in the Rome match that Djokovic repeatedly hit much weaker shots off his BH when faced with heavy, deep, high topspin balls, and Nadal was able to pounce on these weaker shots, many times for outright winners. This happens in every match between them.
Point is, Djokovic may be able to handle high topspin to his BH better than most, though I still think Del Potro can handle the high BH shots better, but Djoker is still affected and thus it's still an oft used strategy by Nadal, hit high deep topspin to BH. I'm not saying Djokovic does not have a first class BH or anything, just that the mechanics of the stroke and sheer physics make it impossible not to be hurt by these shots, even if its relatively less than most.
I think you missed 6 important matches last year + AO this year, all of which happen to be finals, I suggest you watch them