It's a good tool for learning natural languages especially if you cannot practice with native speakers every day.

I agree with you that for knowledge you need everyday it is better to just learn on the job. The relevant knowledge will stick/you can note it down in a cheat sheet if needed. There's just not enough time to learn the full feature set of a programming language, you won't need most of it anyway and you will be a better programmer by reading existing code and writing code yourself, which incidentally is similar to how best to learn a natural language (listening and speaking)