Homepages are the most valuable real estate in the world. The homepage is also your company's face to the world. Potential customers look at your company's online presence before doing any business with you.
Complexity or confusion make people go away. Of course, all other aspects of bad web design should be fixed as well, but if the homepage doesn't communicate what users can do and why they should care about the website, you might as well not have a website at all.
That's why homepage usability is so important and that's why so many books are written specifically about this one topic. There are ten usability mistakes that about two-thirds of corporate websites make.