I got an email at work from a customer, wherein he said what a relief it was to correspond with someone who knows how to communicate like a businessperson. Having seen some of my coworkers' attempts at professional emails, I can only commiserate.
Here's the thing....
I've heard from many an aspiring writer that they feel they don't need to sweat grammar and spelling. That's what editors are for, right?
No, not really. Copy editors clean up your manuscript. They don't hold your freaking hand. And their job is not to do your job for you. (I've seen grammar so bad I couldn't figure out what the person was trying to say. The sentence made no sense.)
But publishing industry aside, there is an even better reason for you to learn how to communicate effectively and professionally: it's how you get people to do what you want.
Yes. Put your Machiavelli hat on for a moment.
If your email sounds like a 3rd grader wrote it, I'm probably not going to pay much attention to it. Your ineptness may even offend me, because it's really easy to get the wrong impression when you're reading a letter as opposed to talking to someone. Your lack of punctuation sounds rushed and even annoyed when I read it. I'm less likely to like you if I think you're being snippy with me.
In short, if I think you're a tool, I'm less likely to do what you're trying to get me to do.
Face it, you only write an email for two reasons: to convey information, and to get someone to do something you want them to do.
Written language is the medium through which you accomplish those things. Therefore, just as you might take a class on argumentation in order to improve your ability to get people to do what you want, you should also learn to use written language in the same way.