Before I had a better understanding of what APs are (I've never had one, we save up and go once every two years), I thought they were only valid for non-vacation times. I suppose more like what the lowest levels are and that, if people wanted to go during vacation times (weekends, summer, Holidays), they had to pay the same price everyone else was paying for entrance.
I'm sure there are all sorts of pros and cons to having the system be that way, but I imagine it would greatly reduce the crowds. Discounted entry (via AP) for people going during what was previously the slow times, so they could keep a steady flow of visitors and income, but charge the same for all guests when they have no problems with bringing in the crowds.
Even though we have to save for so long for a trip, I would rather have to save a bit longer, pay a bit more, and not have to deal with such huge crowds.