It's very handy for e-learning. I currently make ePubs for iBooks. iBooks is arguably the best ePub3 reader out there at the moment. It will render its own proprietary format (books created with Author), but it will also render ePub 3.
The e-learning advantages: You can embed quizzes, flash cards and videos. It doesn't rely on internet access for presentation. Once you deliver the book, be it through download or sideload, the user is untethered. It's easier to develop in HTML5 + CSS3 than Objective-C and Java. You don't have to reinvent the wheel - all of the font and theme controls, bookmarking, notes and highlighting are handled by the reader.
I understand dedicated readers worrying about the evolution of the ePub format into what amounts to a game plugin, but I believe that for content that is primarily text and media with a few enhancements, this is the way to go.
Walrus Books have pushed the envelope with their release of Lovecraft's "Kadath". This book is very interactive, and may cross the line into "game", but it gives the fixed-display ePub3 standard a real workout. It's only sold through iTunes, because only iBooks can render it properly. (see it in action at Vimeo: http://vimeo.com/43381264