Yes, I believe that it is valuable to know and practice the conventions of formal English.
Even though English is our primary language, there are still varying degrees of how it is spoken and written. When we are with people we know in a casual setting, we tend to spend without regard of how our words are arranged as long as we get our message across. However, there are instances in life wherein the knowledge of conventions of formal English will be put in practice. These instances usually happens in formal settings like applying for a job, speaking with a dignitary or person who is ranked higher than you are. This practice of formal English shows our respect and also calls out respect to ourselves. It also conveys how serious we are on the subject matter.
