But can they really a substitute for a good demo?

Well, no, but like you said yourself, it gives the costumers another idea about the quality of the games.

Very interesting stuff about the future of promting games over steam were revealed in an interview of Gabe Newell to “Game Informer” in GDC, the following part is regarding the DoD:S free weekend

And so far everything that we’ve found is that every time we do one of these promotions they’re really effective. And they’re a much more effective way to spend our marketing bandwidth than through traditional means. So, yeah, we’re going to do a lot more of these. Will we do “every weekend’s a free weekend?” Probably not. But we’ll find other ways.

One of the things that we really want to do is help people to get their friends to try our games. That’s a high priority right now for us is to put all of the pieces in place. If you bought a copy of one of our games you can give five trials to your friends. You type in your e-mail address and then you’ll see the first time that they play you’ll have this little screen where you can say, “Oh look, he’s playing. Oh, he hasn’t played yet so I’m going to delete him and replace him with somebody else.” And give your friends a chance to play. It’s a multiplayer game. You get notified as soon as they start playing so you can jump in. That’s the kind of thing we think is going to be super valuable at making that connected customer experience more effective.

The full interview (lots of interesting stuff there:
http://www.gameinformer.com/News/Story/200603/N06.0324.2249.59492.htm