Yes, it certainly is the dictionary definition. The problem is that the dictionary definition of legitimate, legitimate though it may be, is incomplete because acceptance and recognition are being assumed to refer to some authority. So of course it is legitimate to ask who is the authority, and who decides that it is to be recognized. The real answer always must boil down to, "I, the person who is using the word and assigning legitimacy in this context, am the ultimate authority, in the context of what I'm saying." You are choosing what is a good source of authority, whether you are claiming to represent some establishment or not.
Not to suggest that there are no conventions about good sources of authority, or that such conventions aren't useful. But when you use the word legitimate, you're assuming that everyone else respects the same authorities as you do. And it's not necessarily the case, especially when it comes to something as non-standardized as names.
English "rules" are so fine-textured and exception-riddled that it's more reasonable to call them conventions. Since you say "standard" has a meaning that needs to be preserved, I'll propose that "conventional" is a better choice of word to describe name usages that you don't think of as legitimate. It's more accurate because it admits that a judgment is being made by someone. And it is more objective because the judgment is not implicitly being made by you, as when you use the word legitimate. Investing authority in "a mass of educated speakers" is too vague and subjective.
I believe American people google "legitimacy" of names because what they are really looking for is to prove that the name they like is not going to be sneered at as "not a real name." Or else they are trying to avoid names that might be sneered at. They're looking for judgments, not facts. "Legitimate" requires some judgment, some looking up to superiority, and you admit as much when you equate "not being seen as odd / from an uneducated background" with "success." (I'm not saying I don't agree with your judgments, but I don't agree that they are adequate for determining name "legitimacy" for academic or any other purposes.) "Conventional" directly refers to general acceptability. Distinguishing between conventionality and legitimacy makes you refer to yourself as a certain cultural authority, which no speaker can legitimately do. Yes, legitimacy is relevant, but it belongs on the Opinions board, as they say around here.
Or maybe you might modify the word legitimate to make it mean more precisely whatever it is you think it means. "Academically legitimate"? I don't know. But I think "legitimacy" applied to names does not serve your purposes well, at least in the American English that I speak. Unless your purposes include to deprecate unconventional naming.
This message was edited by the author on May 8, 2010 at 10:43:20 AM