Well, that's notorious that the "Coke vs Pepsi" commercials benefit to both companies. When Coke "attacks" Pepsi they're actually helping the idea that the choice is between the 2 companies, and other don't exist. That's the same idea behind the "Mac or PC" stuff.
The user feels he has choice because he has 2 possibilities, but this fake choice is destroying any chance for a third competitor.
