The idea of a demo is to provide a demonstration of the game so that people can get a feel for the graphics and sound and how the game "feels" for the player. Also a lot of people suffer from motion-sickness. The only game that made me sick was Doom3. But alot of people get sick and the older you get the more prone you are for motion-sickness.
When you provide a Multi-player demonstration of the game then you run the risk of people not buying it for one of many reasons such as...
Why buy it when I can play this demo? This is good enough.
This map is terrible, if this is representative of the game then I'm not buying it.
The problem with game demos is that they are sometimes so short that the player feels ripped off even though they didn't spend a penny; for instance that FEAR demo. That demo has scared me away from the game because I suspect that the expansion pack is also quite short.