The truth is that most of us can make ourselves understood when we're speaking. Our grammar might be highly imperfect, but we make up for that with tone of voice, word emphasis, pauses, gestures, facial expressions, etc.
When we're writing, though, we don't have those tools at our disposal. Proper word choice and punctuation are a writer's tools for making her point. And I want to choose my words very carefully here, for I want to make an important statement. Here it is, straight from an English teacher:
Good grammar is not the point.
Really. As much as I love to discuss English grammar--and I do!--I must remember that it's simply not the point. Good grammar is a means to an end, not the end in itself.
So what IS the point? Communication.
When I taught grammar to middle school statements, I made them memorize this dictum:
Language is a gift from God, given only to humans, for the purpose of communication.
Have you ever thought about that? All kinds of creatures communicate with one another in various ways, but humans alone have been given the gift of language. It's a special gift, a tremendously valuable gift. It's valuable because it serves the purpose of communication. Human beings can communicate with one another in vast and powerful ways. Not all communication involves language, to be sure, but a great deal of it does.
In years past, most communication occurred via the spoken word. But we citizens of the age of the internet communicate mostly via the written word. Every day we have opportunities to communicate with others. Every day we have chances to make ourselves understood. Every day we can blow those chances if we don't use our language well. Use of the wrong word or lack of proper punctuation isn't really a big deal in and of itself. But if your readers can't understand what you're trying say? That IS a big deal.
Taking the time to use language correctly is, at its heart, a sign of respect for your readers. If you make a mistake, don't fret about it--just correct it and be glad you had the chance to make the correction. Perfection isn't the goal. Communication is the goal.
In short, the question is "Does good grammar really matter?" My answer: It matters if you have something to say. If you have a word of cheer or hope or light, your use of grammar matters. Because the world needs to understand your message.
A word fitly spoken is like apples of gold in settings of silver (Proverbs 25:11).