If they arent good enough to play after the demo, don't take them. Its not complex.
Ah, but if you've read my infamous post #31, you'd know that demos are unreliable. If I had based my impressions of Hitman: Blood Money off the demo, I would have never bought the full game. Why? Because the demo was a training level that was not at all representative of the game's full gameplay.
You may not realize this but piracy is often a good way for obscure games to get attention. I first learned of Tribes when a friend gave me a copy. In fact, the whole Tribes community was founded primarily on pirated copies. However, we all grew to love the game so much that we all inevitably bought it. That = good demo.
So, for the fiftieth time: The best demo is the final, full version of a game.