And therein lies the problem, you have instructors essentially creating their own set of rules, basically because they can. No different to an ATO saying you cannot use a backup EFIS on an IFR ride because "it is not needle-ball-airspeed".
Now, I am not for one minute suggesting that a flight test candidate should be permitted to point to the little blue dot as an answer to the oft-asked "Where are we?" question, and would go so far as to say that the blue dot should be turned off during the test, such that you are only using it for a track line and chartage.
But, an approved electronic copy of the charts is legal, and should be assessed as such. "Oh, it got too hot/battery is now flat/you spilled your waterbottle on it", then whip out the backup, or program a Direct-to into the panel-mounted GPS. If they won't let you use it, the ATO has, essentially introduced simultaneous, unrelated failures which, IIRC, they are not supposed to do...Otherwise we might as well make those V1 cuts even more interesting in the multi-checkride and pull
both mixture levers to ICO instead of just one...When was the last time an instructor took your paper charts away from you during a flight-test?
I have no problem with DR, paper charts and calculating diversions the old fashioned way. But for an instructor or ATO to prohibit an other-wise legal tool "just because" does not sit well with me, anymore than the ATO ordering you to use charts only purchased through his pilot shop would sit with anyone else.