The issue I think is this: At 180 days out, you have the dining plan, you want a meal, are you willing to gamble NOT reserving with an ADR and hoping that you can get a spot at The Plaza as a walk up guest? If they are full, and the wait is insane, then are you stuck with a set of TS credits you can't use? My guess is that most people are not willing to take that chance, and so they make an ADR elsewhere.
We did have lunch at The Plaza last summer, but we were not on a dining plan. Since we were paying out of pocket, it really wasn't a big deal, if there was a long wait there, we would have simply gone to a QS location for our lunch.