The issue is commercial. The mere existence of PS3 games proves that, as the PS3 was very hard to develop for, but it was worth it because the games would sell.
Steam makes possible for developers to target Linux and sell the games there, in an easy convenient way that is easier to use than pirating games via torrents, but with some DRM to prevent uploading the purchased game to torrents just after the purchase. Ubuntu also has a store that sells games, I can see at the very least: Bastion, World of Goo, Amnesia, Braid, Space Pirates and Zombies. I don't know if they use any DRM, I guess the answer is no.
The final success of this endeavor is also of commercial nature. The number of games sold in Linux will determine the future of Linux gaming, and nothing else will, no matter what the technical merits of Steam in Ubuntu are.