Interesting variation on the the automation debate. I like to think about it the same way. If the aircraft has it you should be able to use it. If your have high end automation then you should be able to operate it quickly and accurately but be able to hand fly it just as well.
Having been forced by my instructors to be able to do mental DR I am eternally grateful. There is a time and place for a calculator (mainly flight logs where accuracy counts and you have time to go heads down) but mental DR is pretty handy. The FMS is not always right and not every approach can be fully programmed to touchdown so the VNAV requires some mental backup. Thanksto my instructors who insisted on mental gross error checks my occasional cock ups have not ended in tears.