Today's article in the Wall Street Journal, "This Embarrasses You and I [sic]" (here), is a perfect illustration of why good writing is so important. Bad writing (poor grammar, poor spelling, poor analysis, verbosity) makes you look dumb.
In today's job market, where there are far too many lawyers trying to get (or keep) far too few positions, you need to remember the three things good lawyers do:
1. They're good at analyzing problems and coming up with viable solutions.
2. They communicate well.
3. They're ethical.
That's what you need to be a good--maybe eventually great--lawyer. Fancy law degrees are great, but they'll only get you in the door. Your own performance is what keeps you there.
Here's a comparison for you to remember: I want my surgeon to be able to communicate well with me. Surgeons cut. They think about cutting; they cut; they think about the cuts they've done. Writing isn't a job requirement, per se, but I sure want what my surgeon writes in my medical files to be accurate and understandable. If my surgeon didn't know the basic rules of grammar, I'd start obsessing about what other gaps he had in his education. And lawyers actually are supposed to write well, so it's even more important for you to prove that you know the writing rules.
To steal a line (or to do a jazz riff on it--I couldn't find the exact quote) from Oprah: be your best self.