Historically, the term "atheist" was used in contexts where:
- there was one predominant religion with one god-concept.
- religion was often ritual-based, where a person either participates or not; there is no middle ground.
(And as a corollary to the last point: it means that a baby wouldn't have a chance to "behave like an atheist" until the age where they were expected to actively participate in rituals).
... so historical usage isn't that useful now, where:
- we recognize that theists who reject a society's predominant god-concept but accept some other god-concept(s) aren't atheists.
- we acknowledge that many theist's religions emphasize faith and de-emphasize practice and ritual.
Unless we're going to insist that all atheists are unreasonable idiots, defining atheism in terms of rejection of deity became no longer possible once there stopped being a "default" societal god-concept to reject. Around the time we stopped calling the West "Christendom", we also undercut the idea that a person could become an atheist just by rejecting Christianity.
While I'm not sure about your characterization of the historical context, I agree that the historical usage is no longer useful. The modern usages don't strike me as useful either, however. As I've probably said before, terms "theist" and "atheist" on the whole don't strike me as useful descriptors. Defining oneself in the negative is particularly not useful, as it doesn't tell me anything about what the person is about. On the whole, really knowing what a person is about takes some pretty long conversations anyway, and isn't going to be distilled down into a word or two, yeah?