Maybe it's because I'm stuck with my AT&T two gig/month plan, and I don't personally know anyone with an unlimited plan that wasn't grandfathered into it, but I have to imagine there's a lot of people with lower-end data plans who couldn't use something like this. I wonder if the Amazon player has, say, a bitrate option to help people use less data, but I would be surprised if it did.
Everyone's talking about moving data into the cloud and assuming that everyone has a high or unlimited data plan, when, in my personal experience, very few people do.