I think it all depends of the crowd you are targetting.
RPG players will play the avernum demo to its end, where they'll see a nagging screen and feel the great frustration of not being able to progress further. Considering the scarce offer in RPG videogames, they'll probably decide to buy it to support the author.
OTOH, Soccer moms and retired computer aficionados playing a Bejewelled demo, need to be reminded that a lot, because you know, their attention span is much more volatile. That explains why they play Bejewelled in the first place