I'd agree that pages without a title banner or heading are probably making a usability mistake, but I disagree that titles don't impact usability. When done correctly, correct titles make a site much more usable. Usability doesn't just apply to the page that a particular user is on, but to the site as a whole. For starters, I thought the article's example of bookmarks was a decent one.

Maybe I'm not the typical user, but I personally find page titles extremely useful when using a back button. I get annoyed when I expand the back history and just see the site name for every entry. I've also noticed a trend in browsers like Chrome that use the title as an autocomplete hint in the address bar.

Also, beyond the SEO importance highlighted by the article, if you have multiple pages indexed in a search engine, it makes it much easier for an actual user to locate a particular page of interest. As a side note, I am actually more inclined to click a search result if the title looks like it was made for me, seems to match my intent, and looks like it was not just constructed for a search engine.