I think this concept has been seriously over-hyped recently on the DIS. I don't thinnk it's a swisespread as some DISers would like to think. Everytime someone posts about it being hard to get an ADR, someone else blames it on double bookers. the truth is that withthe new dining plan, restaurants ahve just become more busy. Seriously, if the problem was really widespread, wouldn't WDW just estimate an average and overbook? I mean think about it...how often do you get seated right at your ADR time anyhow? IF WDW KNOWS that 20% of folks aren't going to come ona given day, then they will just overbook by that amount. If the place has to stay open an extra half hour some days, then it would.
The reason to attempt it would be to have the option that day of going to either meal, but as someone pointed out, DIsney is trying to crack down on this sort of thing.