First, I don't have to keep looking for ids/classes across multiple files when I'm maintaining/modifying an object. This makes maintaining an object that much easier.

Also, because Agility allows me to refer to the root element of a view as the selector "&", I don't have to worry about maintaining that additional id/class. This becomes especially useful when you have many MVC objects (as most projects do), each with their own root element. Suddenly you have eliminated dozens of unnecessary ids/classes.