There's plenty of places that have just one aid (NDB) plus RNAV. Sure, if you go there in IMC you will probably pick the RNAV approach to fly before the NDB, but try going out to some of these places and see what it would be like legally meeting requirements if the NDBs were removed and your delightful machine of choice is not TSO146! On top of that, what about non-IRS equipped IFR en-route position fixing requirements? On top of that, I'm sure I'm not the only one to have had GPS do funny things at inappropriate times. Always nice to have a ground based aid to revert to.