Is it relevant?
I think so, it's basically putting a name to an approach used by the CSS author, in much the same light as creating styleguides. Is it useful? Yes. Is it easy to inherit someone else's OOCSS work? Probably not.
What are the benefits of OOCSS?
Abstracting style attributes of a certain component is always good for style consistency over the whole of the site. Say you want to change the border style of the components for a fresh new look: You'd typically change a few lines concerning the border-style master style.
An example
I created a UI Developers styleguide (or style vocabulary) concerning all the 'widgets' in our web app. Each widget would be classified according to its intended contents. Each widget could have any number of predefined box-styles, background-styles. Each widget could also lay out its contents different depending on what parent element it was placed under.
This amounted to code like:
