I know that they at least USED TO teach in school that "ain't" is not a grammatically correct word. Its use has increased nonetheless. But even if it has been gramatically sanctioned by now, "ain't seen nothing" is still a double negative. making it incorrect English. Another issue to consider is whether the word "ain't' in its increasing acceptedness should be allowed to cover the territory of "haven't" as well as that of "isn't", "aren't" or "am not". The usual colloquial function of "ain't" is as a negative form of the verb "to be". But in a few phrases, like in this song's title line, it becomes a negative form of the verb "to have", since the standard way of stating the line is "You haven't seen anything yet", not "You aren't seen anything yet". So it is fair to question, however acceptable "ain't" becomes in lieu of "am not", "aren't" or "isn't", should it be further stretched to sometimes mean "haven't" or "hasn't"?