Yes, the waterproof cast is TOTALLY worth it!
Our DD broke her arm DAYS before our trip (and her birthday). It was right at the elbow, so her cast went all the way up her arm. It didn't inhibit our trip at all, and frankly, the attention she got from the characters and other CMs more than made up for anything she couldn't do (which wasn't much).
As you can see, the cast went all the way up her arm. (It was Super Bowl weekend, and we're from Seattle....hence the crazy cast!)
Suzy and Perla were VERY concerned. She got lots of hugs and kisses from the characters. This was pretty typical of the interactions we had.
The only rides I completely nixed for her were Goofy's Sky School and the Matterhorn. I just wasn't comfortable with the jerky side-to-side motion on those. I figured that if my arms hurt after I got off a ride, it was a no-go for her. On Mater's, I made her sit so the cast was next to a person instead of the side of the car, because that one has a LOT of side-to-side, too.
Space Mountain was her most favorite!
Splash was down on our trip, but I would have let her ride (waterproof cast). It was only in the mid-60s when we were there, so no one wanted to ride GRR.
Honestly, she did EVERYTHING she wanted, even Jedi Training. It was a little hard with just one functional hand, but she had a great time!
I was totally freaked out when it happened, and very concerned about how it might impact our trip, and it was really a non-issue. We had a great trip!