You have a point. But as a non-native speaker trying to learn a language, you aim to become so fluent that people will not notice you're foreign. You want to be able to play with the language.

A big part of learning a language is to become familiar with frequent speech patterns and slang. A language is not a sterile set of words with attached grammar but a slippery gelatinous blob that molds itself to the culture and people. Spoken languages are quite lively. If I want to integrate myself and joke around with natives, I need to learn to mold it the same way as natives to. In order to learn how to do that, you first have to start imitating.