Now, I had to be the guy that has to bring this up, but demos are supposed to be the things that you play in order to decide if you're going to buy the game at all. If you're already sold on the game, wouldn't you want to wait for the full product instead of playing a small chunk of it ahead of time, and then replaying that same bit when you get the real deal?
Or are you so busy fiending for Left 4 Dead that you'll take what you can get, when you can get it? I suppose getting the demo in the hands of someone who's already on board gives them extra days to convince their friends to buy. Either way, it sure sounds like Valve is anxious to get the games into the hands of players. Take a look at this related quote!
“Every time someone plays Left 4 Dead, we witness the most overwhelmingly positive reaction that we’ve seen from any of our games,” said Gabe Newell, president and co-founder of Valve. “This gives us tremendous confidence in the product and why we’ve taken the game on the road, produced this pre-launch demo, and invested more advertising dollars on the title than any prior release from Valve.”
Weirdly enough, I've managed to not actually play any L4D yet, but I don't think I've heard one person say anything negative about it so far, though most of the people I've talked to about the game seemed to agree that you'll need to play it with a competent group of friends in order to truly enjoy it.