I’ve now checked out that WaPo article that @northwesty flagged in #231, read a bit about some of the schools he’s been discussing and I think I’m starting to understand this a little better.
First when you look at the chart in the article showing the schools that admit the highest percentages of their classes ED, the first thing that jumps out at you is that the vast majority are LACs. This says to me that LACs in particular feel the need to lock down a lot of their classes with ED because they’re subject to so much competition from research universities and each other. Also, as @northwesty says, it increases their percentage of full payers and gooses their perceived selectivity (and therefore their competitiveness) by reducing their admit rate.
The next thing you note in that chart is that the research universities there tend to be the ones that are trying to move up in the rankings, e.g., Penn, Emory, Tufts, Vandy and Northwestern. This, I think, is one of @northwesty’s points, in that he argues that they’re accepting all these kids ED in order to look more selective. But, as @JHS says, if these schools are trying to move up a notch, it’s also logical that they’d try to pick off kids who might ordinarily aim higher. The best way to do this is to lock some of them up with ED, as these schools seem to be doing. I also still believe that increasing the proportion of the class you accept ED is a natural response to the growing numbers of kids cascading down from higher up the food chain, which is related to @JHS’ point (and to what @menloparkmom is saying, in that you reduce the potential yield management problem you face if the absolute number of seats left for you to fill is smaller because you accepted much of your class ED).
Finally, with regard to the virtuous-because-they-don’t-have-ED USC, Georgetown and Notre Dame, my hunch is they don’t have it because they’re troubled much less than most other schools by the need for full payers and perceived selectivity. Why? Because, it seems to me, unlike the universities in the previous paragraph, they occupy particular market niches that give them the full payers they need without difficulty, they’re not subject to the same competition as their peers and they’re not trying to move up the hierarchy because they’re happy where they are. USC is where you go in CA if you want a top school that isn’t a UC or LAC and Stanford’s out of reach - and the student body is 20% legacies, most of whom, I would imagine, are full payers. Georgetown and Notre Dame, both top-25 schools, are arguably the leading Catholic universities in the country, top in that category in their respective geographical regions; Georgetown is the oldest Catholic university in America, with some highly regarded specialized schools; Notre Dame, which has a $10b endowment and where nearly a quarter of its students are legacies, probably doesn’t have a yield management or full-pay issue. Working the ED angle wouldn’t give these schools anything they want, I believe.