What you also have to remember at WDW there are MANY international travelers, whose "holidays aka vacations" are not always at times those of us from the US might expect. Homeschoolers, year round school schedules, etc all make it very difficult I think to try and schedule around the crowds. We have gone at times when I just could not image there being many school age children, and I am surprised every time.
Obviously there are the BUSY times, and the times that the crowds are lighter, but it has been my observation especially in this past year that the crowds are always heavier than I would have expected. Two weekends ago we so SOOOO many school groups (many of them international travelers) - one group made us laugh -they were all dressed the same in BRIGHT yellow T-Shirts and pants - you could not have overlooked one of them if you had wanted to..... it was a large sea of yellow moving around EPCOT@
Just my observation/experience in the multiple visits we have made to the parks (we average at least 3-4 visits per month Sept - May and leave the parks to the tourists in June, July & August!!!)