It got me thinking. Learning a language by talking with real, live natives is one of the best ways to learn. But you should keep in mind that not 100% of what you hear will be "technically correct." And it can be difficult to pick apart local dialect and vernacular from incorrect Japanese. Thus "real" Japanese is not always "correct" Japanese.
A good example is the way 「全然」 (ぜんぜん) is often used. You may hear someone say something like 「全然いいよ」 (that's perfectly fine). However 「全然」is correctly used with negative forms. 「全然良くない」 (not good at all, or completely bad), for example. Using it with a positive like 「いい」 is technically wrong.
Of course languages evolve and change over time, and oftentimes speaking fluently means making mistakes and using "incorrect" words or phrases.
I guess it all depends on your goals. If you're aiming to reach a conversational level and that's it, stuff like this is purely academic and you can ignore it. But if you're aiming for a job, say, that requires you to be able to write and speak in "good" Japanese, or if you're studying for a proficiency exam, this kind of thing is worth being aware of.
Personally this is why I think textbooks have merit and are good supplements to immersion and other forms of study.
What do you think? Is knowing "correct" Japanese important?