My constant use of the phrase "grammar and usage" must annoy some people. Certainly the locution is unwieldy. But most people do not understand that matters of grammar are not precisely the same as questions of usage, and I cannot bring myself to perpetuate the general confusion.
The grammatical structure of a language encompasses the classes of words, their inflections, and their functions and relationships within the sentence. "Usage" refers to the conventions that govern the way the language is used in a language community. Standard written English, the form of written English accepted by the educated writing and reading community, is governed by a set of conventions (rules of usage) that must be mastered by anyone who hopes not to be dismissed as inadequately educated. In some ways, the rules of usage serve as a class marker, dividing "us" (the "educated" reader or writer) from "them" (the "uneducated" or "imperfectly educated" reader or writer).
If you doubt that there is an element of class consciousness inherent in questions of usage, just listen to the things people say when they run across what they consider to be an error in usage. Don't a lot of them seem a tad snotty and condescending, as if the purpose is not to help the speaker or writer, but rather to put him in his place, to let him know that he is not quite up to "our" standards? (I don't want to be unfair here--sometimes the comments just reflect the frustration of one who has spent years perfecting his own use of the language and who resents the apparent laziness of those who have not, yet who expect to be received as if they had.)
I can't approve of la-di-da snootiness, and yet I can't approve of carelessness in writing, either. I would not want to attend a play where the actors haven't bothered to learn all of their lines, or a concert where the musicians figure the audience won't mind their lack of virtuosity as long as their hearts are in the right place. I love language. I love writing. I love reading. If you want me to read what you write and to value what you have to say, then please show me that you care enough to at least try to get it right. Taking care with your writing is like wearing the right clothes to a dress-up occasion--it's a sign of disrespect to show up in a torn workshirt and dirty sneakers.
Just the other day, a friend who was in my office while I responded to an e-mail from my sister exclaimed in disbelief, "God! I can't believe you proofread and correct your e-mail!" You bet I do!