Well my guess is the Alola forms will still count towards the National Dex, getting the original forms would just be a matter of personal completionism. However three in particular (Raichu, Exeggutor, Marowak) seem particularly hard to get the originals for outside of transferring them in fully evolved since their previous forms (Pichu/Pika, Exeggcute, Cubone) don't have Alola forms, so even ones transferred in might evolve into their Alola forms instead of their originals.

Feels like quite an oversight with these three, unless the National Dex somehow fixes this