Like it or not the majority of the ways we communicate in technology is through the typed word. Email, Twitter, Word, these are the methods by which we argue a point or ask a question. Our communication skills form impressions with our clients and coworkers.
All too often, however, we also set ourselves apart from others with our writing. Not because we excel at it, but because so often we make mistakes that stand out. Many technical people aren't good at getting their point across because they either don't focus on their topic, or they make lots of silly writing mistakes. Grammatical mistakes don't have much to do with your technical ability, but they seem to give you less credibility.
I ran across this blog from Brian Kelley that talks about setting himself apart because of his writing skills. As one of our authors here at SQLServerCentral, I concur. I rarely need to edit anything Brian sends in. In this piece, he also links to an article from CopyBlogger on silly mistakes.
Writing is a skill, and it's one that you can learn to improve. However it takes practice. It's one of the main reasons I recommend that technical people blog. You will improve your writing skills (if you work at it), and possibly impress someone else who could offer you a job.
Learn to write better. It doesn't take a lot of time or effort, and it will pay off for you throughout your career.