GPS should be used secondary NOT primary, what did you do, when the GPS signal failed ? apart from pray
The same could be said for DR, what do you do when you realise the you've been following the wrong railway line for...oh I forgot to note the time minutes.
Primary, secondary, makes no difference. The basic emphasis of navigation is you sensibly use what data is available. If you are IFR, regardless of whether a TSO'd GPS is used or just using a VOR to track airway, you would be daft not to use NAV2 tuned to another VOR (or the same if unavailable). Why have an ADF not tuned to anything that will be or is within the DoC, etc.
The same goes for VFR, if you have a chart on your lap and you prefer DR, why not also switch on the GPS (if available) and cross-reference. If you use GPS, why not use the chart and eyeball pre-determined points. Which is primary and which is secondary? Who can say, I guess what is important is that the pilot has organised himself such that his work load is reduced and the chance of erring is reduced and the chance of picking up errors is increased. Simply saying GPS must not be used as a primary aid means very little IMHO.