It was a real proto, TV Sports Baseball is out there as well. No one has dumped those games though.
Fantasy Star Soldier is floating around too and unlike the HuCard games above, it is locatable.
Kinda reminds me of people who buy famous paintings and put them somewhere where nobody can ever see them. What's the point of such things existing if not to be enjoyed by people?
If you think about it, the majority of recent "most valuable" items are in the hands of collectors, and not museums. I don't think a single museum owns an Action Comics #1, Nintendo World Championship Cart, or a Honus Wagner Baseball card. I think it takes people passing away, and donating collections to museums for these valuable-type items to become part of museum collections.
The point being, (sadly) that a part of collecting is just "keeping-up-with-the-Joneses." Having a prototype that hasn't been widely released has more value than one that has been made available. Therefore, they feel "better" than you, because their collection is nicer.
There are numerous prototypes sitting in people's personal collections, that the world just won't get a chance to play. A couple that come to mind are Marble Madness 2 (arcade), and Socks the Cat (SNES). None of the owners of these are willing to release ROMs, because that puts a massive cut into the monetary value of these collectibles.