I totally understand what you mean about rain ruining a WDW trip. We went 2 years ago in late August and landed as a tropical storm circled over Orlando for most of our trip. We hadn't been to Disney in 3 years and booked an 8 day trip. The constant rain ruined our trip for several reasons:
I didn't get a tan, constantly cloudy, colder, and windy. When you were outside you wore a poncho that didn't cover all of your clothes, so your lower legs, and feet were still wet. When you entered a ride that had the A/C set for a 110 degree day.. it got pretty cold and miserable, pretty fast.
Any outdoor attractions (specifically at Universal Studios) were shut down due to the storms.
Disney is about enjoying the atmosphere of the parks and different lands. My friends and I really liked just walking around and looking at all the details of the parks, especially at night. When it was a pour down you would move from one indoor location to another. It really cut the trip in half.
Water parks were only so much fun when the sun was rarely out - I go to the parks to have fun, but also get some sun. A storm can really ruin the day and also shut down the parks when it would lightning, etc.
Ultimately, I hate the rain at Disney.. not just a few hour storm, but when it would go ALL day and night. It ruins most outdoor shows, parades, and activiites. And you're right, we had saved up and looked forward to that vacation for a long time and it was a huge bust! Luckilly we have several other vacations that were much more improved after that!