I think the point is because not all Indonesian people use nggak. People from eastern Indonesia stick with Tidak or Nda for informal. Enggak is basically in Java, Sumatera and Bali. And if you use that one in formal conversation, people will think that you are uneducated.
That's a good think they teach you first official Indonesian. As some people who takes course here could want to pass exam in Indonesian. So, it's very good to be able to make the difference between the official Indonesian language, and the slang.
If I learn English on a website, I'm very happy they don't let me write "nite" for "night" or "wanna" or "gotta".
It's good to learn very well first a language, and when we have all the basis, going into slang, not the reverse, as we could mix everything.