While the actual ATP test itself is a bit of a joke (really just the instrument rating again), the main idea behind the ATP requirement is the 1500 hour part. While not every hour of experience is necessarily meaningful, and while there are plenty of 250 hour geniuses who are better than some other pilots with more hours, as a general rule, more hours = more experience = more chances to see unusual events that you can draw from, to guide you through new problems that you have never encountered before. There's nothing better than scaring the crap out of yourself to ensure you don't do stupid things in the future. Sure there are exceptions, but as a rule, the more experienced the pilot, the safer the pilot. I know I was safer at 1500 hours than at 250 hours.