Grammatical rules are akin to dress codes: They were once dogma, but now are in a constant flux.
The Washington Post reports on the Associated Press's style guide's final acceptance of the word hopefully in the sense of "it is hoped." This appears to be the beginning of the end of one of the longest-running cultural battles, between professional linguists (who study how language is actually used) and language mavens, who establish rules of good usage.
The ban on the optimistic hopefully has always been one of the least defensible taboos. After all, to my knowledge German grammar authorities never condemned the cognate hoffentlich, though I'm sure the mavens would counter that the French language uses double negatives all the time. They might be perfectly logical and understandable in English, too, but they still sound like words used for effect, as ain't is now.
The mavens-vs-linguists controversy reflects one of the great trends of the last hundred years, the weakening of authority by diffusion. Professionalization probably reached a peak as a movement in the years just before the First World War. When I was reading a historical medical journal on line for another project, though, I found a growing alarm by some physicians that their patients were emboldened to challenge their advice, behavior much rarer before the War. Language mavens seeks authority for themselves, but they also question the validity of academic linguists in appearing to condone just about anything grammatical, and defend middle-class values against campus relativism. Some linguists, for their part, relish the role of freeing the populace from the oppression of what they consider the false logic of the mavens.
In the end, usage really isn't related to grammar or logic but is a realm of fashion. And this cuts both ways. Just because something is, linguistically, grammatical English doesn't mean it's expedient to use it. It's like wearing jeans or a suit. Clothing tastes, like grammar instruction, were once rigidly prescriptive, too. The tragedy of the General Slocum, which caught fire in New York's East River in 1904, was compounded by the heavy woolen clothing that helped doom many passengers in the water—though the excursion took place on June 15. Dress codes, even in luxury restaurants, are in flux.
What this means is that in language and in clothing, there is no single standard any more, except at publications that rely steadfastly on a style guide and have the resources and skilled copy editors to enforce it. Often the issue is not the garment or the word, but how the wearer or user carries it off. But just as we no longer have columnists with the broad influence of Walter Lippmann, neither mavens nor academic linguists have given us so far a master arbiter like the English writer Henry Watson Fowler. A battle appears won, but the conflict goes on.