He was worse in April and July than he was in September last year - he just had an all-around mediocre year. But even with that, he had an OPS more than 0.100 points higher with runners on base than without and kept driving runners in.
True - but then my next question would be: if he consistently hits the alternative style of pitching better than the normal style of pitching, why wouldn't you just pitch him normally? There are a few players that consistently have the higher numbers, but more players tend to average out over their careers. For example, Berkman's numbers are absurd this year with a 1.200 OPS with runners on. But last year, he was worse with men on than without and over his career, there's not much differential. For Abreu, he's substantially better every single year. I have no idea why, but it tells me that players can hit better in differing situations - so why can't one of those be "in the clutch"? For Ichiro, it makes perfect sense - his first few years, he was the same. But he hits the ball on the ground so much, so the bigger holes in the infield with people on base become base hits. With Abreu, those types of explanations don't make as much sense.
Bear in mind that Pettitte and Clemens are old, and Nieve's one of the few good young arms in our system. He's got the potential to be a very good pitcher. I've been impressed with what I've seen out of him so far.