The airlines have gotten a lot smarter about their pricing and also about their booking. Where flights with empty seats was common years ago, out of the several flights I've taken in say the last 3-4 years, I'd say maybe 10-20% have had any empty seats at all, and even on those, there were very few empty seats.

This is all just based on anecdotal experience, but it seems that they have really figured out how to manage their inventory while maximizing the dollars per butt-in-seat. In the end, I think it means more predictable fares and that the gap between the lowest paying passenger on the plane and the highest paying passenger is smaller. My guess is, it also means that, while you may sometimes find the best fares 60-90 days before the flight, on balance, I would prefer to buy as early as possible and not take the risk of the flight filling early. That's just the way I'm looking at it lately.