Speaking with GamePro, series creator and director Cliff Bleszinski explained:
A beta is like hooking up with a girl just to say, "yeah, I fucked her." I know that sounds crude, but it's the honest-to-God truth. Once you play a beta, you can check it off your list--you can say, "yeah, I played it." Then you might not feel motivated to get that initial cherry popping from the proper, final game.
Granted, we may need to tweak a few things in multiplayer after release through an update. People are going to find exploits we didn't anticipate. That's the reality of making a game -- look at MMOs. We're prepared for that, but we think we're shipping a pretty great game out of the box here.
The Xbox 360-exclusive action game sequel arrives worldwide this Friday, November 7. In related news, Treyarch just opened the Call of Duty: World at War multiplayer beta to all Xbox Live Gold members.