Yes, and should the HDD go poop or can't get sufficient power at some point, such a user might complain. Hey, I can have my HDTV plugged into my outlet NOT grounded, and I can go along for quite a while--until it really matters, then it is COOKED. Hey, run your tires UNDER minimum PSI with a good brand made today, you can chug along for quite a while before it blows.
I suppose if the box is not powering too much (but that could change), one can get away with a bus powered HDD, but why spend the time and energy saving all that content with the risk of failure when Dish has been explicit about using an external powered HDD, hey, all the MSO's (cable, TiVo, DirecTV) require it as well because they can't guarantee their STB's will always be able to sufficiently power bus powered HDD's. . Using an HDD with its own power supply will provide the necessary power no matter the system configuration. I think the idea is to eliminate as much risk to allow the greatest likelihood of preserving your saved programming. To do otherwise is asking for trouble.