We make ADR's because we don't want to have to stand around for 45 minutes (or whatever time it might be for a non-ADR party to be seated) when we are hungry then. We also have very specific places that we want to eat and specific times, so we do it to make sure we get what we want. If we didn't care, then we might not make ADR's. We generally do signature restaurants for dinner and if we don't eat breakfast on the fly, then it's a character meal. We also do the Castle as well as Akershus for the princesses. During NONE of our meals at signature or character dining places did we see empty tables. They were all full; or in the case of the first ADR time of the morning, they were being filled one right after the other.
I think it depends on the restaurant and the time of year for sure. We only walked into one sit-down place on the fly and that was Sci-Fi. We planned a counter service lunch and I was just so tired of "junk" food as well as being limited by what our kids would eat, we decided to see how long the wait would be for Sci-Fi. We got a nice table in about 3 minutes. One other family walked in right in front of us and got a table, too. It didn't look that empty, but I do think there were a few empty tables way in the back that were smaller.