There are many expressions and grammatical constructions that are not normally used in Standard English. These include regional expressions, such as might could, and other usages, such as ain’t and it don’t, that are typically associated with dialects used by people belonging to less prestigious social groups. These nonstandard varieties of English are no less logical or systematic than Standard English. In this book an expression labeled nonstandard is not wrong; it is merely inappropriate for ordinary usage in Standard English.
It is important to remember that formal and informal refer to styles of expression, not standards of correctness
. Informal English has its own rules of grammar and is just as logical as formal English. You can be serious using informal English, just as you can be comical using formal English. The two styles are simply used for different occasions.
Introduction. The American Heritage Book of English Usage. 1996