Immersion is not always ideal. Sometimes you want to make an effect that is anti-immersive. Sometimes you want gamers to not feel like the world is believable because that immersion into the game world would get in the way of something happening off screen.
Will Wright uses Guitar Hero as an example. He believes immersion is not a good thing for that game to have because it would distract players from enjoying each others' maladroit plastic guitar heroics.
But of games by which we make virtual things do virtual things in a virtual world, like an avatar in an RPG or units in a strategy game, immersion is typically effective, desirable. A world that seems real leaves a powerful impression on gamers. This is not to say that a game world should mirror the real world to the point where the fun feelings from a game are negated; although there are gamers who connect electrodes to their nipples.
I can't think of anything else to say. Your thoughts?