the septims where Dovahkiin and as the game introduces you to in the beginning is the fact that only the emperor could wear it. however as we all have seen, good old mankar doesn't really care about the rules.
I think that the septims where Dovahkiin was essential for wearing the amulet so in theory all dovahkiin can wear it. and there by our good friend Mankar has to be Dovahkiin to.
http://images.uesp.net//a/ad/OB-npc-Mankar_Camoran.jpg
Anyone agrees?