RNAV and NBD Approaches are two different animals, just like VOR/DME and RNAV are two separate approaches. The reason they still maintain VOR and NDB's and the FAA certifies and publishes these approaches is that GPS is not a required element for IFR navigation and is not yet mandatory. Not everyone has an IFR approved GPS. Just installing a GPS in a plane does not give you the authority to fly IFR with it. It must be installed and approved for IFR navigation and IAP's. We have a turboprop twin that has a GPS installed and approved for enroute IFR navigation but not for approaches. If the approach is a NBD Approach, even if there is also a GPS approach (NBD/GPS) then you still, must have the ADF unit installed and operable to legally execute the approach. You cannot legally commence that type of approach until all elements of the approach, including the NDB station has been tuned and verified as operable. But... Until the X-Plane design team decides to create a workable GPS, even if its something on the line of the MSFS Default GPS or until add on makers can make a GPS for both PC and MAC versions of X-Plane with a level of function close to the real unit (Reality XP Style) there will be a need for ADF's in the sim. To me the one of the biggest short comings to XP is the default GPS. Heck even the default MSFS GPS blows XP's GPS out of the water. Reality XP is not supported for Mac users. That said, I feel that we are fast approaching turning point with X Plane and the quality of add on's across the board. All you have to do is just look whats offered up here and whats coming in the next six months.