You make such a valid point here! You can have a website that is completely optimized for the search engines, sending tons of traffic your way, yet if it is not usable, it won`t convert anyhow! Usability is #1 in my books. Before I will even optimize a website for a client I do a Usability Study for them and we incorporate all those changes first. This may include enitcing sales copy that funnels your user down to the buy button or it may involve recreating your navigation to make is easier to find what your use is looking for.
Search engines are all competitors and they want the best websites to show up in their users queries for their users. They want the sites people love and enjoy using. They want the best usable websites to be #1 so it stands to reason that if you build a usable website, that is easy to navigate with themed content that a user is looking for, you will rank at that top of the search engines eventually.
Its only a bonus to optimize your title, redo meta-tags. clean up code plus some other SEO processes but the whole secret to SEO is making a usable website that the users love. This will make the search engines love you too and rank you high!