Is Grammar really important?

I've always wondered if English teachers have laid too much emphasis on Grammar and its correct usage. Really, how much of your elementary school Grammar have you used in your day to day life? As long as what you conveyed to the other person is comprehended, how does it matter if you used the right Grammar or not? Many Americans use a lot of slang in their language and they never had a problem driving home their point. A puritan might disagree by saying that the way you express yourself indicates the way you think. Not quite true! How can you judge a person's intelligence by his usage of language?

I'd think, the only place where Grammar comes into play is when you are writing a book or explaining something legal which depends on the all the t's getting crossed, i's getting dotted, the precise usage of pronouns and prepositions to make sure there is no way to misinterpret.

Flow in your comments and have a great weekend.