The whole point of SEO is to gain traffic and get people to stay on your site so they can be entertained or buy your products and services. As such, SEO very much goes hand in hand with usability, because this is what will make a difference in whether or not someone stays on your site for long. If your site is hard to use or navigate, it is very easy for people to go to the next search result. Also, the search engines themselves will look at layout and usability. If your site is hard to navigate for your viewers, it will be hard for the crawler as well, and having a bad usability can definitely affect your rankings.