Summary
Using WAI-ARIA states to communicate changes in the DOM provides styling hooks that are as useful and easy to use as standard HTML classes. Although purely preference, I'm also a fan of the fact that an entirely different selector is used to communicate state in the style sheets; it is simpler to spot within a rule.
None of those prior factors really get you anything new. What using WAI-ARIA states will do, virtually for free, is start to provide a better means of communicating your web application state to users of assistive technology. If money talks, consider also that by using WAI-ARIA you are widening your product up to a greater number of users (see the additional info below).
As such, using WAI-ARIA states and properties, is the recommended means of communicating state in projects adopting a ECSS methodology.
