They reused Naxx because the initial requirements to enter it in Vanilla, and the level of difficulty it was tuned to, severely hampered the amount of people that actually saw the instance.

As for reusing Nef and Rags, I seriously doubt it's going to be a copy/paste of their previous selves. I would imagine that they're going to use some of the old abilities just as a tip of the hat to Vanilla-raiders, but they'll likely be completely new encounters. And reusing a single boss is far different from reusing an entire instance (no one complained when they reused Anub'arak from Azjol-Nerub in Trial of Champions).