They do need to finish the game sometime this year, after all. 

After all the anticipation and hoopla over the impending Halo 3 beta, many fans scratched their head at the studio's decision to only host the public beta for three weeks. In the PC world, these tests are typically much longer, and Bungie's Frank O'Connor addressed these concerns in this week's update:

    "...obviously the number one reason is that the longer we're spending resources taking care of the Beta, the less time we're applying what we've learned from it to the final game. And statistically, three weeks is going to seem like a pretty long time when you look at the hours you log on the game.