As much as consumers may like demos, all the data I've seen shows that having a demo actually decreases sales of games, which is why most games don't have demos anymore.
Unless your game is actually good, in which case I'd think it would drive sales. Anecdotally, I've bought several after trying the demos for them (and the opposite as well).
There's certainly precedent among some of the larger players. Look at Blizzard - just about everything has occasional free weekends or trials of some sort after the initial release.
It also prevents buyers from feeling cheated. I'd rather play a demo and realize the game's just not very good, than shell out $50 for it and more or less instantly regret it, with very limited (if any) options to return it at that point.
There's certainly precedent among some of the larger players. Look at Blizzard - just about everything has occasional free weekends or trials of some sort after the initial release.