How much is a kilowatt hour? How many KwHs does your refrigerator use? Or your furnace, or front-porch light? How many gallons of water do you use when you shower? How many gallons of gas do you use to drive to the grocery store? Yes, a MB is an abstract measurement that is hard to visualize, but so are many other measurements we use every day.
The biggest problem is the arbitrary caps, and the problems in trying to see where you are at any given moment. In theory you can go out to the water meter or Electric box and get a reading to see where you are, but even those are difficult to understand if you don't have a reading from the billing date to compare to.
The real issue is that with today's technology, if you don't ASSUME that someone is going to pirate your work (and come up with a business model that works around or with that) you are essentially just "giving it away and praying".
I like Wil's work a lot, and would buy the hard copy of his book just to support him, but he is essentially just saying "Here is my business model, if you go around it, you are a dick." This would be equivalent to me trying to sell a book for $10 in Washington State, and $15 in Oregon, and just calling everyone in Portland who drove across the river to save a few bucks "Dicks".
Today, there is no amount of DRM that is going to prevent piracy, it won't even slow it down much. Even only releasing hard-copies will only last until someone gets bored. You HAVE to make free downloads a part of your business model, whether you like it or not.