Part of the reason Disney changed their cancellation policy for many restaurants is that there were many people who were scheduling multiple reservations, and not cancelling at all. This left some restaurants with empty tables that could have been filled by last minute reservations and/or walk ups.
People still overbook restaurants, but they'll be much more inclined to cancel, at least the night before, in order to avoid being charged.
I definitely have more ADRs than I'll end up needing for our next trip. I made the ADRs for tentative dates around 180 days in advance, because I knew that some of them would be more difficult to get after we finalize our plans, whenever that is.
So, yes, it's very common, and worth it to check for cancellations up to and during your trip.