But all the times I've rolled my eyes as southeastern US accents, or the classic Quincy no-R's pronunciation, or the crude farmers' Portuguese of my ancestors...
According to my (thus far) favorite textbook:
...adding Standard English as a new language or dialect involves much more than learning grammar, vocabulary, and syntax. It requires the expansion of one's personal, social, racial and ethnic identity to make room for the new language and all that it symbolizes and implies.(p. 44)
Mutual intelligibility is often cited as a criterion to test whether two language varieties are dialects of the same language. However, this test does not always work.[...]languages such as Spanish and Portuguese are mutually intelligible. Yet they are classified as separate languages. In these cases, political status rather than mutual intelligibility plays the deciding role in distinguishing a language from a dialect, thus the assertion that a language is "a dialect with an army and a navy".(p.40)
Finally, a beautiful quote from African American author James Baldwin (1924-1987) rounds out the awesomeness provided by my textbook reading today:
It [language] is the most vivid and crucial key to identity: It reveals the private identity, and connects with, or divorces one from the larger public, or communal identity...To open your mouth...is (if I may use Black English) to "put your business on the street": You have confessed your parents, your youth, your school, your salary, your self-esteem and, alas, your future.
PS. I feel that I should add a sort of disclaimer: I'm becoming a teacher of Standard English. Why? Not because my students' languages are any better or worse than mine, but because learning SE will open doors for them. It will also make them bicultural, bilingual, and those are good things, if difficult. I love Standard English; it's my language, no better or worse than theirs.