Baseball is the leading example of why giving away most of your data is the best use of it. The sport of baseball -- the way it's played on the field, the way players are scouted and trained, and the way it's enjoyed as a fan (Moneyball? Fantasy Sports?) -- have been revolutionized by amateurs making use of free open data.
If you give out the great bulk of your data, people will be enhancing it with metadata, building tools on top of it, and most importantly connecting it to the rest of humanity's knowledge store and mining it for connections you'd have never conceived. Giving out "up to last month" or "daily intervals" will grow sharply the market for "real time" or "second-by-second". Baseball's mission statement concerns bats, bases, butts and seats -- not visualizing correlations among heterogeneous data stores. By releasing their data for free they let the smartest people in the world have the opportunity to perform that second task for free.
We're about to enter the age of ubiquitous information. Drawing these data stores into open formats, making them discoverable, and interconnecting them across knowledge domains presents explosive opportunities. But who will own this data and what access will they allow? If you want to help ensure that the answer is 'everyone' and 'all of it', come join the http://infochimps.org project, a free open community effort to build an Allmanac of everything.