Tropes Are Not Bad [http://tvtropes.org/pmwiki/pmwiki.php/Quotes/TropesAreNotBad]. Plot twists are useful, especially in games, which normally take a helluva lot longer than most creative media to consume. Compare to a TV show, that's done in 22-44 minutes, or a film that's 90-132 minutes long, a game's standard 8-hour playthrough can seem really long. And there are times where a changeup is needed, lest the player get bored. One way is to vary game mechanics; another is to introduce changes to the narrative.
It can also be used, as in the Bioshock games or Spec Ops: The Line, to instill a sense of cognitive dissonance, and in the process hopefully teach the player a valuable life lesson.
Of course, the key to a properly-executed plot twist is to foreshadow, but not telegraph the twist too early. The audience/player must, at the time of the reveal, feel like everything suddenly made so much more sense in hindsight. Case in point: Bioshock's twist may seem out there, but if you managed to collect the right recordings and read between the lines of some main character dialogues, then you suddenly start to see that the author/designer had been trying to inform you of the twist much earlier in the game. Since entire threads have been dedicated to analyzing the Bioshock endings, I leave it to far better men than myself to explain everything in detail.