They have an effect indeed (Score 1)

Of course they have a massive effect. But it really depends if the demo showcases the games best aspects or not.. Demos that give you the first or second level is not really showing you the games potential because we all know the first levels are the "Hi, Your dumb because your new so let us explain everything that is already obvious but sadly there are still people who don't get it". Simply if games showed more of the massive rule bending aspects of the game then demos would benefit.