When I was planning for my trip over November and early December, I had a couple of near double bookings (one at 4:30 and one at 5pm, for example). I was very concerned that something would end up canceled, not by my doing, because I had read about such on discussion boards. Now, I never double booked with the intent on keeping both until my actual trip. My reason was because things do fill up quickly, and I was planning as fast as I could, but as a newbie, it was quite difficult to immediately plan everything out and have the whole trip itinerary set in stone. And I'm an extreme planner! And I had done a couple years of research before booking my trip!
Due to my hubby's hesitancy, I booked only 3 months before my trip. At that point, some ressies were already hard to come by. I booked what was available as soon as we had our hotel booked. Then I went back through and set our itinerary, and then called to cancel the couple of double bookings. That process took about a month, maybe about 6 weeks. I feel I that I moved as quickly as possible while planning, and then was courteous as well by cancelling the doubles as soon as possible. If I had waited until everything was set, I wouldn't have gotten half the restaurants I wanted to try. I know a lot of people are WDW veterans and you know what you want to try and where you want to go, but for newbies, it takes some debating and second guessing and more research to finally nail things down. Having a couple a double bookings helped me relax just a little while I could think things through and make a final decision.
I may be splitting hairs, but I too agree that double bookings should not be allowed during one's trip. BUT, during the planning process it is helpful to have the ability to do so. I cancelled well before my trip and thus someone else was able to book my previously booked spots.
Maybe Disney could institute a system where double bookings are tolerated until, say, 30 days before check-in? At that point you have to decide or Disney decides for you, thus freeing up the ressies for others? I know others disagree, and that's okay. But it would give planners time to iron out details, but prevent those who abuse the system from repeatedly double booking just so they can have ultimate dining flexibility during their vacation.