Developers are also users
As a software developer I care deeply about usability. And in my book, the term "usability" is more than just shiny new widgets on a shiny new website. To me, the usability is just as much about finding the appropriate api abstractions to program against as it is about delivering the best and shiniest UI to our customers. It is all about user experience. If you're a user of an api, the experience of using this api should be just as smooth and effortless as using the best designed desktop or web application...