the size of the software in those original NES games has got to be from 8kb, to maybe a MB or 2 at most (I can't imagine any of them over 1mb). Even if the new device has a 250mb drive with it, I would think you could easily fit 400 games on it, possible even 500 (the majority of those NES games are probably under 500kb each, with ALOT of them significantly under 100kb).
They can better recreate that true NES experience with their hardware/controllers, and make the device, in essence plug and play (ease of use). Emulators can be cool, but may require some setup/config, you might be using a controller that functions the same as an NES controller (but isnt the same). For me, emulators have always had some kind of limitation that is annoying, it might be a small limitation, but annoying nonetheless.
I know Nintendo will not allow this, but if the new device allowed adding other ROM's from external sources (usb, internet, etc.) it would probably render most, if not all other NES software emulators useless. (btw, they will never allow non-certified ROM's to be played, its just a pie-in-the-sky wish from me)
Considering this is going to sell for $60, and I'm a lifelong gamer that was in the 7th grade when NES came out, I can't see a scenario that I won't drop the 60bucks to get it. Plus after looking at the game list, pretty much everything I wanted to see was on that list, so it seems like a no-brainer for me.