I agree, I'd rather have them than not, especially in the parks. If you are there during a busy time, then they're almost a must. And, for places like CP at the MK, the last 2 times we were there and one of them wasn't even that busy, they were turning down everyone that didn't have an ADR. Even the Plaza gets busy during busy times.
I wish I had one on the day we went to MGM in December. It was so packed and still a week before Xmas. The counter service lines were 20 deep!! We ended up getting a pretzel for lunch, how I wished I had an ADR that day (& I usually do, but for some reason forgot to make one for that day--or even wished I packed a sandwich!!)
If you know what park you'll be at it's worth making one somewhere because you can always cancel.