I am based in the Nuremberg region and I use NUE as much as possible. The reason is that it is one of the easiest airports to navigate, has a great airport hotel (Moevenpick), efficient check in, a really nice lounge, and the security often takes only a few minutes. The feeder flights to the various hubs are fairly frequent and well synced with the transatlantic flights (at least if you're on Skyteam or fly Lufthansa). This makes the airport experience much more pleasant than MUC or FRA. While you can reach both of them in about 3 hours using the ICE (plus subway), if you are already in NUE, it's really not worth the hassle of the train ride, the much higher cost of the airport hotels, and the inconvenience of the much larger airports.