Set style mirrors 'competitive' play a bit more, and adds a bit of challenge to the game by not allowing you to switch your Pokemon immediately after an opponents Pokemon faints. Thus, switching your Feraligatr into Golem is a bit riskier, because Earthquake/Stone Edge from him will hit hard.
CPU doesn't switch nearly enough (very seldom) in these games. It would be much more interesting if they'd pull their Water Pokemon away from an anticipated Thunderbolt, bringing in your immune Ground type.
Maybe next generation!