The answer to the question of which games even allow 4K is:
Basically any modern game engine. Fixed resolutions being pre-defined is mostly a staple of old games and indie games. When PC monitors went from 4:3 to 16:10, developers of game engines mostly addressed that problem by just detecting which display resolutions are currently available to the system and offering those.

My statement about where this card would even be useful is:
I'm tired of the cyclical argument people have about current need. If something fast or big is released, it's labeled useless by the same people who would be chastising the creators for taking so long if it were currently necessary.

That being said, I personally would find it immediately useful for Battlefield 3. I've yet to play that game on hardware that gives me acceptable performance the way I want to play the game.