It's valid mostly for the fact that only Nords and Imperials have exhibited the ability to be Dragonborn. Not to mention the ability to shout. Seeing as they are the ones who made the pact with Akatosh to gain this ability. It seems to be a distinct ability only those of Atmoran stock are capable of.
Canon wise I would say the Dragon born is a Nord. Same as how the Nerevarine is more than likely a Dunmer. Where as the Hero of Kvatch is probably an Imperial. Though he could very well be a Nord or a Breton... For some reason I'm leaning towards him being a Breton.
But that would be fairly lame for many of the people who play Elderscroll games if they had only Nords for Skyrim