This article over at Gameplayer examines the gaming tradition of unlocking extra features and modes.
"I mean that in the literal sense – forcing you to unlock game features slows things down. I also mean it in the figurative sense – it's stupid. Granted, it can sometimes be used as a logical, intuitive means of imparting a sense of achievement in the player. Yet unlocks are often used as a crutch; an attempt to inflate the importance of game features that otherwise would have little value."