The real answer is that ideally you're getting enough sleep in a block to not need a nap. The reality is that a lot of people can't due that do to life circumstances, work schedules, or health issues. So often enough a nap is better than powering through the day on insufficient sleep.
The reason I made the thread is because I was reading an article from the National Sleep Foundation linked earlier that said that even with seven to nine hours of sleep a night, most people will feel tired between 1 and 3 p.m., so it looks like it's not just a matter of not getting enough sleep at night or doing too much during the day. Sounds like that's just how we're wired.