This is far OT, and I can feel eyes beginning to roll already. My apologies to everyone who's sick and tired of this perennial discussion; you can stop here. But ...
First, I didn't say learners should learn the most common usage, but the most standard usage. The two are by no means always the same: think of lie/lay, or misspellings like *seperate and *definate, or mispronunciations like, well, *pronOUnciation, or *mischievIous. If we judged solely by vote count, the wrong one would probably win in a lot of cases. The same is true of other unpopular rules, such as taxes or speed limits. There's nothing sacred about pure democracy, and nothing inherently wrong with letting a better-informed elite make some decisions for the community. The trick is choosing which decisions, and keeping a sensible balance.
And making sure that the so-called elite is actually a relatively large group. The people who determine language standards are by no means just a tiny junta of dastardly prescriptivist 'usage experts' imposing some artificial set of rules (except perhaps in the case of the French academy or the Neue Rechtschreibung committee *g*), but rather everyone who works with language or simply loves it for its own sake. Writers, teachers, editors, readers, translators -- we may be a statistical minority, but we're a significant and valuable one, enough to exert a certain natural authority.
Of course, authorities in some fields have more authority than those in others, and in the extent to which they have to defend their authority, language mavens are probably more like, say, cops or climate-change researchers than like pilots or neurosurgeons. Language isn't a matter of life or death, everyone knows something about it, and a certain range of opinion is indeed possible, indeed desirable.
For that reason, the concept of 'ownership' of language may work better in smaller linguistic communities. I do believe that it's better for everyone if language lovers 'own' the standard written language. The community of people who communicate by writing, via letters, books, magazines, and newspapers, rightly influences the broader community as a whole, including foreign speakers wanting to learn the language. I don't see that changing even with the upsurge in electronic communication; if I'm wrong, I hope I won't live long enough to know about it. (-; But those people don't own other subcommunities at all, such as teenage slang or txt msgs or blue-collar language or technical jargon or regional or ethnic usages, and that's entirely as it should be. There's a place for every kind of language, and in some places, standard written English is simply wrong.
And yes, if the weight of public opinion eventually becomes overwhelming enough, fans of traditional standard language will indeed have to concede the occasional lost battle. But in my view, a 3:1 (AHD) or 4:1 (Google) ratio is still very far from that point. So I would say we can keep preferring 'graduated from' with a clear conscience. (-: