Bethseda games are actually one of the perfect examples, because you see... the point of Bethseda games, isn't the game itself.. it's that it's a low barrier to entry game engine (we're talking between $20-50 after all) allowing you to create mods to do almost whatever you want, which is ultimately the real point of those games, and they do have their unofficial patches, complete texture overhauls, models, quests, landmasses, etc.
And then we've got things like Planescape Torment, which while it didn't provide an SDK, it has it's own unofficial patches and mods to restore the original content, and finish it up, and otherwise change it.
To be very blunt here... If a game it popular it will be modded , no ifs, ands, or buts. So to say that opensourcing the game engine is useless completely fails to comprehend the PC gamer community.