Someone who needs easy mode might start on normal, get to some point where the game becomes impossible, then switch to easy. (Speaking from personal experience here.)
Smart games will do this for you. For example, Warcraft 3 initially lets you choose between normal and hard, but if you lose a mission, it unlocks easy mode. This way, nobody will end up playing through the game watered-down unless that is the only way they can make it through. Everybody who turns out to be competent will experience the game as intended, regardless of their initial estimation of their abilities.
Left 4 Dead's director does this on the fly in-game.. Having and easy time? It throws more at you, less items. Too hard? Opposite.
It's actually a lot more sophisticated than that. Here's a great presentation from Valve about the "AI Director" in Left 4 Dead: