DW and I are both teachers

in different districts, so summer is our only option. BUT...that's only been for the last 5 years. Prior to that, it was by choice.
Summer heat is unbearable for some, but the great thing about Florida in the summer is pools and AC.

We choose summer because we know it will be hot, beautiful, and the pools will feel great at the end of the day.
Before I was teaching, we had gone in the fall, winter, and in April and May. When going to Disney, I expect to wear shorts and a t-shirt to the parks. At all times of the year, we had to wear long pants and even jackets. Disney just isn't the same with cool (or cold) weather.
Besides, if you follow the advice of people here on these boards, you hardly notice the crowds. One 4th of July, we saw more in the first two hours of the day than on any other visit. The MK opened early and many people avoided the "morning rush." DD practically had Fantasyland to herself. Last year, we visited 22 attractions by 4 pm (in late June) and had plenty of time to revisit the favorites.

It's not about the crowds in the summer, its all about knowing what they're going to do and scheduling around that.