Why does Jose Valentin switch-hit?
Jose Valentin is a switch hitter. Against right-handed pitchers, he hits from the left side of the plate. During his career, his OPS in these situations is .821. Last year, it was .879. Against left-handed pitchers, he hits from the right side. During his career, his OPS in these situations is .585. Last year, it was .599. In these situations, he hasn't had an OPS of even .700 in any season since his rookie year, when he only had 19 at-bats. Just to make clear how significant the difference is, here's his career performance from each side, prorated to 650 plate appearance (approx. 1 full season):

	H	2B	3B	HR	RBI	BB	SO	Avg	OBP	Slg	OPS
LHB vs. RHP	144	33	4	30	93	66	129	.253	.332	.489	.821
RHB vs. LHP	116	20	2	9	51	59	151	.205	.281	.304	.585

Valentin has played most of his career at shortstop, so let's compare those numbers to a couple of shortstops of his generation. Miguel Tejada's career OPS is .822. Rey Ordonez's career OPS is .599. So, from the left side of the plate, he has hit like Miguel Tejada, and from the right side, he's hit worse than Rey Ordonez. You'd think this huge gap would be enough to convince Valentin to stop switch hitting, and simply hit from the left side against all pitchers.

And in 2005, Valentin was finally convinced. He hit from the left side the whole season. Against southpaws, he had his best season ever, with an .891 OPS! He only had 19 at-bats against lefties, but it still seemed pretty convincing. Despite his unprecedented success, in 2006, he went back to switch hitting. Predictably, he did poorly against lefties, with the aformentioned .599 OPS. In one at-bat last season, he decided to give hitting lefty against lefty another chance, and he hit a homerun! But, then he went back to switch hitting.

Why does he continue to insist on switch hitting? It seems insane!

Hmmm, here's one possible explanation. Over his career, Valentin has been very consistent from the left side of the plate. His highest OPS season was .880 and his second lowest was .758. The one outlier was 2005, the year he ditched switch hitting, when his OPS in such situations was .546. Maybe Valentin thinks that suddenly facing lefties from the left side somehow hurt his timing against righties.