If you think a demo will ruin your experience upon release, don't get the demo.
Consumers are much less likely to purchase a product if they are unsure of it's quality. If they don't release a demo, the people who are skeptical of the Bull Shots and odd canon breaking things they put in are not going to purchase it. If they are able to get there hands on a demo and find that it is fun, they'll buy it.
AvP is a VERY multiplayer game. People will be getting it for the purpose of playing online. I think it would be a good idea to market to the multiplayer market.
I'd love a singleplayer demo, but I think a MP demo would allow me to get a better feel as we would be able to play as each species.