No not everyone watches the extras...but if they were to not be included with a digital purchase/rental then they'd never be watched. Not many people would pay additional money to get extras online, I know I wouldn't.
As it is it's already value-added to DVD's to make the product more attractive, especially for the repeat sales of the same movies. It's how they milk extra cash...releasing a new package with more content every few years. Seeing as this business model has been VERY successful for film companies, it's proof that people DO want extras.
Good lord, could you imagine the 4 disc LoTR sets as HD downloads...over 200 gigs at least. Not going to happen.