I agree for IR renewal it should be aid only, as GPS can (and do) fail.
However it is perfectly legal (not to mention easier and less stressful) to set up your GPS in a omni mode and fly the NDB approach as you would a VOR approach on your HSI. As long as the NDB ident is going and you don't go outside of the 5 degree tolerance on the ADF that is perfectly legal.
I know on some PC12's this is the preferred method as the ADF get's some shielding during turns on approaches.