Predictive and Non-Predictive Stats
Several years ago, Voros McCracken stirred up a hornet's nest with this article on Baseball Prospectus' web site, which drew a line between the numbers a pitcher does and does not have control over. While most of these were clear-cut (strikeouts, W-L record), McCracken asserted that earned runs and hits allowed were largely in the hands of the defense behind the pitcher.
With some reservations, everyone in the baseball analysis community has accepted McCracken's conclusion that hits allowed and ERA are a poor predictor of future success as a pitcher when compared to defense-independent pitching statistics (DIPS).
Of course, this doesn't mean that major-league baseball teams have listened. Russ Ortiz, who for years had put up terrible DIPS numbers despite above-average ERAs, was given a $34 million contract by the Diamondbacks before the 2005 season. Ortiz was released, with full payment of his remaining contract, less than halfway through the deal by a team that valued his empty roster spot more than a pitcher they had recently valued at $8.5 million per annum. He continued his failures with the Orioles, who signed him for the major-league minimum.
As a whole, baseball teams do a poor job of recognizing which statistics will yield future success and which will not. Fifty years from now, baseball historians will look back and laugh at the idea that teams and fans of this era used save totals as their primary evaluator of relievers, laugh at how the Mariners and Yankees were fooled by the career years of Adrian Beltre and Carl Pavano, laugh at how teams were still overpaying for pitchers whose lack of strikeouts made it clear they wouldn't repeat their success.
(Warning: NSFW Earl Weaver link)
Thirty years ago, Earl Weaver understood the value of players who would put up numbers that will always win you ballgames: walks and homers. Whatever team signs Juan Pierre for way too much money this offseason is going to learn an expensive lesson about what his skill set is really worth. Meanwhile, Russ Branyan can't stick with a team despite being an above-league average player for his career.
Smart baseball teams profit from the knowledge that plate discipline, making contact, and power are the paramount skills for hitters, and commanding the strike zone and keeping the ball down are the keys for pitchers, and that almost everything else is just fluctuation. Most of the current terrible contracts in the league would never have been given out if teams kept this basic concept in mind.