Also makes it so people play more. Some people can't stand not 100% finishing a game, wich is good for microsoft.
Example:
Someguy: I got all acheivements accept for overkill.
Someguy(Next day: Now to go and finish that acheivement, dang! My XboxLive ran out! Go buy more months so I can finish the overkill.
And Microsoft gets 50 more bucks.
Voted Best Nazi Mod Ever <3