In my arabic studies they actually made us "choose" a dialect after the first two years. I couldn't believe how different some of them can be from the Standard Arabic!
The idea of "choosing" a dialect fascinates me -- I studied Spanish, and it seemed that with each and every lesson, professors would say offhand that this particular usage or this particular grammatical structure would sound commonplace in one Spanish-speaking country and absurd in another. It seems to me that learning one specific dialect would prevent a non-native speaker from sounding a little ridiculous everywhere. When I imagine a non-native American English speaker switching between northern and southern expressions, it sounds sillier than if the speaker would commit to one single dialect.
How often do language students choose specific dialects in language acquisition? Who else has experience with this method of learning?