Nadal is an incomplete spin player for me - he only has one wing (forehand) where he can get good spin. If he could hit a massively heavy ball off the backhand as well he'd not need to give up so much court trying to hit forehands way into his backhand corner. And then Djokovic's predictable DTL play would not be nearly so damaging.
If you had his forehand, you'd be running around your backhand all the time too. It's the right play based on the effectiveness of the shot. This doesn't make his backhand ineffective or bad. His rivalry with Djokovic entails more mental consistency rather than imposing certain patterns of play. They play the same patterns more or less, just execute more or less effectively depending on the day.