User blog comment:Blitzbear93/Dragonborn Question/@comment-1992639-20121127021145

It was mentioned that the Dovahkiin was made by Akatosh's will rather than having inherited a bloodline. Tiber Septim was known to have used the Thu'um until he lost his tongue, but to me it is unknown if his heirs could speak the Thu'um as well. The later Dragonborn emperors may simply have been Dragonborn in title and family line, not in power.

Dragon Aspect is likely a big once-a-day buff that makes shouts more powerful, drawing upon the Dragon part inside you. I'm guessing you'll need it to actually throw the first Dragonborn around. Or maybe it is a nice extra.

Fun fact: Arngeir mentioned the Greybeards were not sure if there were more Dragonborns currently living. Maybe this was a silent hint from the developers all this while about this DLC.