The television show "Myth Busters" on the Discovery Channel runs experiments on different popular premises.
I am watching the show right now about humid baseballs versus dry baseballs. This applies to the humidor at Coors Field.
They did some really excellent science stuff, and determined that dry baseballs went much further than humid baseballs. There you go.
Colorado is a dry state. A high desert, if you will. If you leave baseballs out in the Denver air, they get dry. If you put them in a humidor, where they keep the same moisture as they would at sea level, they stay moist, just like they do in New York, Boston, Miami, Houston, San Diego, Seattle, etc.
Think about it. A dry apple gets hard. A humid apple stays soft, and tasty. Which would you rather have?
P.S. They also did some experiments on corked bats during the same show. They concluded that a corked bat deadens the ball when it hits it. Ergo, corking a bat is a bad idea.
P.P.S. They also proved there is no such thing as a "rising fastball." A fastball may not drop as much as a different pitch, but it does not actually rise.