From a creative standpoint, it’s the difference between someone reading your masterwork to the end… or until the third or fourth grammatical in as many pages (or less) forces them to move on.
But what about professionally? How about the fact that on Elance, writers who merited 5 stars had nearly half the errors of those garnering 4.7 stars. And nearly two thirds less errors than those with 4.4 stars. As a freelancer, you already know how valuable those stars are toward garnering the best jobs.