I found a really interesting demonstration of why curve balls may be so effective at fooling hitters. Try it out yourself right here. To me, this explanation holds a lot of water. It's not so much that a curve ball is curving that makes it tough for batters to hit--because over time batters can learn to track the path of a curveball and project where it will end up. Rather, it's essentially an optical illusion--a disruption to the optical sensing of the ball's location. It lasts only a split second, but of course even on a slow 70-mile-per-hour pitch, a momentary confusion over actual trajectory and location is enough.
This links up a bit to something I read in a recent issue of Wired Magazine (The May issue, also known as the Mystery issue) where a master pickpocket talked about some of his techniques. He talked about distracting the victim with one hand while making the heist with the other hand. His distracting hand, though, worked best when moved in an arc instead of a straight line. Apparently, a fairly recent manuscript published in the journal Nature (co-authored by a group of magician-type people) discovered through research that the human eyes cannot successfully track an object traveling in a curved path. When, by contact, an object moves in a straight line, we have learned to track its trajectory and then project to the eventual end point. Our eyes, it turns out, automatically move to the end point when we see something in linear motion. However, when an object is moving in a curve, the only way to discover the end point is to continue to track the object with the eyes--the eye-brain connection doesn't allow us (for fast-moving objects at least) to project the end point and automatically move our eyes to that location.
This, too, could help explain why a curveball or slider is so much tougher to hit than a straight fastball.