Yes it would be the third parties that would have to put their old games up for download, but in all reality it would be in their best interest. The ability to make money off a game that they have made no money off of for say 15 years or more. I expect to see a ton of third party games available.
And I too already own every Nintendo system, and many games for each, yet I would still download more games than I could possibly play probably. The convenience of playing them all on one system. Forget the clutter of having my N64, SNES, NES, and Gamecube all hooked up. Just my revolution and and a couple SD cards full of old school gaming goodness. Not to mention the fact all those old consoles, and carts, will eventually bite the dust, more than likely before Revolution. Plus the ability to get hard to find, and nie on impossible to find games makes me happy. As a lover of old RPG's, some of the hardest, and most expensive, to find, the proposition is very enticing.
And what a way to fight illegal emulation.
My first download would be Metal Gear, even though I already own it.