CK Evans argued that the concept of legitimacy was inappropiate when speaking about English names because in most (if not all the English speaking countries) any name can be registered. Legitimate is sometimes synonym of legal (in this case, CK Evans will be absolutely right), but other times has other meanings and one of those meanings if the used when the people talk about legitimacy in onomastics.
I can't speak for all the languages and cultures, obviously, but in the case of the languages that I know in some degree (all of them European) the concept of legitimacy is relevant (that is why "legitimate" and "legit" are used by the speakers). I don't saying that legitimacy is the best word, the best term... just that the concept exists and is relevant.
On the other hand, acceptance is different from legitimacy. Something largely accepted will finish being legitimate, as I said in other posts, but not necessarily. Since the meaning of legitimate which I was refering to includes the concept of "standard", you have to keep in mind that who marks the standard is usually not the most large group, but the more powerful group.