Here's a quick synopsis of the book from Wikipedia:
The central premise of Moneyball is that the collected wisdom of baseball insiders (including players, managers, coaches, scouts, and the front office) over the past century is subjective and often flawed. Statistics such as stolen bases, runs batted in, and batting average, typically used to gauge players, are relics of a 19th century view of the game and the statistics that were available at the time. The book argues that the Oakland A's' front office took advantage of more analytical gauges of player performance to field a team that could compete successfully against richer competitors in Major League Baseball (MLB).
Rigorous statistical analysis had demonstrated that on-base percentage and slugging percentage are better indicators of offensive success.
In short, it takes fielding out of the equation to more accurately look at a pitcher's ability alone. And if you're wanting to really gauge a pitcher, isn't that what you'd want?
Once again, I don't know shit about math or statistics. But since I'm in marketing and advertising, I do know a thing or two about accepting as gospel things that aren't relevant, important, or insightful--and then being unaware of, or even willfully ignoring, things that are.
So Brad, if you happen to read this, thanks for helping to take the blinders off. I owe you one.