Agreed. A download is pure potential. If you have 1,000 copies of a game cartridge or disc then that's all there is. The number can only go down as copies start to get destroyed. With a download, it really works in reverse. You start with one. This single instance of data is then offered around and out of that one instance more appear out of thin air. Essentially, the instances of that data is limited by very few external forces. In theory, every single XBLA title can be as abundant as the Xbox 360 itself. It could even exceed the number of 360s as you can have a computer with multiple copies of the same data. It might not be usable data if it's not on a 360 but it still exists in that instance.
And like I said, there is no appreciable difference between a download that was obtained legitimately and one that wasn't (crackz, hakz0rz, w4r3z, etc.). You can have a copy of the data that was included on the 1,000 cartridges I mentioned above but the difference between the ROM and physical copy is more measurable.
Look at Nintendo's Wiiware games. Without going into specifics, it is very possible to obtain any of those games without buying them from the Shop Channel. What if Nintendo decided to remove Lost Winds tomorrow? Sure, nobody would be able to buy it legitimately anymore. The consoles that do have it legitimately are in Group A. Group B, however, consists of consoles that have the game via download from other sources. It would be very difficult (if even possible) to distinguish between Group A and Group B. And even if you could, few people would care. Any individual instance of the game has the potential to multiply.
So talking about downloads in terms of rarity is like inquiring as to what the rarest SNES ROM is. You can scour websites, newsgroups, torrents, P2P and list out the ROMs that are less available I suppose. But few people if any would be willing to pay a premium for that ROM.
You could think of it like a transaction that can be ongoing and a transaction that's a one time deal. If I found a one of a kind prototype tomorrow I might be able to get somebody to pay me to dump the game and provide the ROM. But the second I hand over that data, I can't go to potential customer #2 and offer, "The ROM ripped from the original game for a fair price." Whether he pays to get the data from me, or gets it from customer #1, or finds it on a website, it's all the same. I suppose in theory you can have a hold out situation where each person is railroaded into paying for a ROM and thus absolutely refuses to distribute it to anyone else without being paid as well but I can't imagine that going on for very long in the real world.