Comment by shmat

Krashen never said that you can learn to speak and write just by listening and reading. He has said that it's a waste of time to speak too early, before you have understood a lot of input. You still have to produce a lot to get better at it. There's also a huge number of studies that show that people who read a lot in their native language speak and write better and studies that show the same effect when learning foreign languages. There's also the practical point that it's useless to be able to speak well if you can't understand the responses you get.