If the reason is so that they won't have to spend time making and locking down a time-unlimited demo version, I can kinda understand that. I don't agree with it, but I can at least comprehend the reasoning.
But why one week? Why not three weeks? A month? You know, enough time for people to actually arrange to get together with their friends on a weekend when everyone is free at the same time, and give the game a try?
They would still get to shut off the demo and force people to buy it if they want to carry on playing.
One week is just ridiculous. I might well have bought it at full price if it turns out to be great, but the chances of me trying out a co-op game properly in such a short amount of time are damned slim, and I'm not blowing that much money on finding out if it's good or not.
One week demo == Definitely waiting for a price-drop.