DLC = pay to play?
If Valve requires players to pay money for 2 Versus campaign that many players feel should have been included with the retail package (especially after not charging for content updates for Team Fortress 2), they're going to (1) lose some of the fan love they've earned with TF2 and L4D and (2) make things much more complicated for groups of players who like to play together:
Player 1: Hey, did you buy Dead Air Versus?
Player 2: Nah, man, I'm broke as a joke.
Player 3: Yeh, I got it.
Player 4: Nope. Don't get paid until next week.
Player 1: Oh. Well, I guess we'll play No Mercy Versus. For like the 10,000th time.
Player 2: Hey, man, don't blame me and Player 4. It's Valve's stupid fault for charging $x for what should have been ready at launch.