Money, and any other type of reward that substitutes for money such as vouchers or access to powerful site features (e.g., features that would otherwise be subscription-limited) encourages deceit and gaming. If you pay me to search, I'll search when I don't need to (just to make money), advertisers will be getting impressions that aren't useful to them, and it'll drive the price of impressions down. Because you can't reward people for the quality of the data they contribute, you have to settle for the act of contributing
something. And once the reward is worth money, you'll get people contributing crap just to get the rewards. Your reward system is now paying people to piss in your data pool. That hasn't happened yet for
Chris Sells's offer to share royalties in return for Amazon reviews, but that's only because of the scale he's operating at. Needless to say, the rules are also different for intranet applications rather than public Internet applications.
