The language that a person learns in school is prescriptive. I think that it is good for language students to learn how language should be spoken.
Learning how a language should be spoken always comes into conflict with how a language actually is spoken. It doesn't only happen with English either. I studeied Russian in college, but wasn't able to speak it very well there until I picked up some of the local dialect.
I studied textbook Japanese, but nobody talks like that either. It is nice to know, but a person comes off as kind of weird speaking it.
I have heard plenty of anecdotes about how academic Spanish is difficult to communicate with in Mexico/ Spain/ Guatemala/ Puerto Rico/ ...
As a second language teacher, I am amazed at how readily slang creeps into the writing and communication of my students. On one hand it makes their English more "normal," but on the other hand it makes them seem less intelligent. I think that it is important for language students to be familiar with more than one register of language to be able to communicate effectively.