How often do you visit a website and find out of date material? Frequently, in my experience. When you do, it makes you less likely to rely on that site for the information you are looking for, or to pick up the phone and call them.
Having a website requires an ongoing responsibility to maintain it. For most companies it is their most powerful marketing tool, so it seems logical that it should be the best it can be. This PR blog explores more about the importance of website content.