Even before I became a high school English teacher, I felt passionately about correct grammar. Strangely enough, I never received strong formal grammar instruction in school because when you were in the advanced English classes, the emphasis was on reading, critical thinking, and writing.
As any avid readers can relate, grammar and vocabulary comes more naturally the more you read because you internalize the rules and words in practical action. I think grammar and vocabulary are actually more effective this way. Although, admittedly, I do not know all the grammar rules and crazy nuances that come with our wonderful English language, I actually do not think they are all that important.
Did the English teacher really just say that? Yes, I sure did. Let me explain though. I value being able to write and speak compelling arguments. How compelling will your argument be if you have incorrect spelling, poor grammar, and limited vocabulary? Not very, you guessed it.