Having a site with terribly constructed, poorly expressed, and error-ridden writing is worse than having no site at all. It’s kind of like those instances when people have an actor or musician of which they think fondly and have always wanted to meet, but when they finally meet the person they turn out to be a real jerk. While your site doesn’t have the ability to refuse to autograph an album or refuse to take a picture, it can still leave a bad impression on the visitor. These negative associations are worse than the visitor never running across your site at all.
Sure, a great design adds credibility to your company or organization, but pair that with great writing and you’ll really have something to behold. On top of making your company or organization look credible, well-crafted writing adds to your expertise and will prove to be more persuasive. It will move people to actually want to buy your product or support your organization. Isn’t this why your site exists, anyway?