in addition to what have already been said above, ain't can be "is not", "am not", "has not" or "have not" depending on the context.

It's because it's slang and not taught in books as proper English. It makes perfect sense though. When talking to friends it's fine to use slang like this. In an office environment it's considered unprofessional to use slang like in your example. Other similar examples: "I ain't gonna lose to you like this." "They ain't got no chance to win." "We ain't heard a word from them."

This is the grammar of the earthy and uneducated. I've never used "ain't" unless I was quoting someone (ie. someone really simple-minded) or making a joke. Really, all you need to know is that it exists, but you'll almost never use it.

Honestly, you DON'T want to use grammar like this because it is absolutely incorrect. The "grammar" is slang (linguistically called "Black English") and really isn't grammar at all - at least compared with standard grammar. "Ain't" really isn't a word in proper English, although it is used as a slang word for: isn't, don't, etc. EX: She ain't comin' out = she isn't coming out. EX: She ain't gotta chance = she doesn't have a chance As for your additional questions: "Ain't" is used here because the person who is using it doesn't have a clue about real English grammar. To compensate for it, they are using slang. "Ain't nobody got time for that" emphasizes the fact that nobody has time, rather than simply that no one has time. Similar to: "It's hot" vs. "It's damn hot." You asked about "nobody got time for that" and "It's not nobody got time for that." If you say "nobody's got time for that", you'd be correct. As for the second suggestion, "it's not" is wrong and doesn't make sense. It's either the slang "ain't" (which isn't recommended) or proper English.

I agree with you that dialects should be respected because there is a lot of history in them. Linguists dedicate their lives to studying how language is used, not just by the elite but by the masses as well. It is important to keep these dialects alive, well, and untainted. However, I feel that it is equally important to keep the standard dialect in tact and for it to be respected as well. Equality for all language forms, without regard to dialect, should be the focus. It's important to me to preserve each dialect, which includes Black English, Chicano English, Appalachian English, Standard English, and the like. So if someone wants to practice a dialect that is different from the standard, that's ok. But it should also be ok for a person to only want to practice the Standard Dialect as well. :)