There's a difference between learning a language and mastering it.
Every little bit helps. Read one book and you've learned a new point of view. Read that book and build a sample project and you've learned more. To really understand a language or framework requires still more work. To master it might take years.
You should master one or two things (or more, if there are enough hours in the day or if you are superhumanly good at mastering languages). But that doesn't mean that you shouldn't also have lower levels of expertise in a broad range of other subjects. Half the battle in life is knowing what to master at any given moment, and the only way to figure that out is to sample a lot of things.
Every little bit helps. Read one book and you've learned a new point of view. Read that book and build a sample project and you've learned more. To really understand a language or framework requires still more work. To master it might take years.
You should master one or two things (or more, if there are enough hours in the day or if you are superhumanly good at mastering languages). But that doesn't mean that you shouldn't also have lower levels of expertise in a broad range of other subjects. Half the battle in life is knowing what to master at any given moment, and the only way to figure that out is to sample a lot of things.