Who's doing the considering? Does God waste his time on making moral judgments about English sentences?
"Correct" or "incorrect" isn't inherent in the language itself. Now, an employer might look at your language to decide whether to hire you, so they have a standard for correctness. That essentially has to do with weeding out people of an undesirable social/economic class: people from the lower classes tend to have less education, which means they haven't been taught all the linguistic conventions of the ruling class.
More generally, I want to interact with people who use language more or less the same way that I do. This has to do both with prejudice (using someone's language as a way to distinguish "us" from "them") and with efficient communication (less time wasted on translating).
The point is this: when you say "that's incorrect" about language, what you really mean is "you don't use language the same way I do, so I don't want to talk to you". Which is fine, but it doesn't justify condescension.
