Then why do they bother keeping such stats? Maybe the heat gets to a pitcher, so he doesn't do as well in July or August as some others who may happen to do better in the heat. If some guy has (theoretically) a career avg ERA of, say, 3.30 in May, 3.50 in June, 4.40 in July, 5.20 in August, 3.80 in September -- wouldn't you think maybe there'd be a reason for it?

It's the same thing as performance before the All-Star Break, and performance afterwards. Some players are better in the first half than the second half, in general, over the course of their careers, than others who might be the opposite way. If you don't find such breakdowns (splits) helpful, then that's your problem. :p

I didn't say that's the ONLY thing I go by - but it is important, whether you think so or not...