This is why I believe having free weekends are much better than demos. You get to play the real game, with the real people. It's much better to judge its quality. Plus, people who already have the game may rejoin to play for the free weekend, knowing there'll be more people around.
Of course, for companies it also means they won't get stuck with people playing demo that's just as the fun as the final game (ie, BF42 demo). It's a win-win situation.