The most compelling reason, I think, that we shouldn’t make fun of dialects traditionally considered dumbed down versions of English is that they, too, are rule-governed, and we might make fun of them incorrectly. And that sort of defeats the purpose of mocking them.
This article talks about three specific examples from dialects that are often made fun of. One is the Appalachian a-prefixing (he was a-huntin’). Another is the Southern use of the word “liketa” (She liketa killed me!), which is similar to the word “almost.” And the last is the African American use of the stressed “bin” (she bin married), which has a different meaning and usage from “been,” with which it’s often confused.
A fun reminder to get off our linguistic high horse (something that I’m often guilty of).