Based on a number of years observing college admissions from certain elite private schools, if you are a full payer at such a school, have excellent grades and scores, interesting ECs (not necessarily to the level of a hook like being a recruited athlete), are a legacy at an elite college, that college has a history of admitting a few kids from your school most years, your family has been generous (meaningfully so, if not necessarily at the level of a “development admit”) to the legacy college or otherwise highly involved there, you apply early to it and there aren’t too many other legacies of the same college applying at the same time from your school, what I’ve seen would suggest that you’ve got a very good chance (although no guarantee) of being admitted - certainly far, far better than the average applicant. Is the legacy preference meaningful here? I don’t think anyone could credibly argue that it isn’t. The point, though, is that it’s one critical piece of an otherwise clearly attractive package.
It seems to me, though, that the value of the legacy is highly dependent on the context in which it appears. In the above example, given everything else and the fact that the candidate is a “connected legacy”, I think it’s very important and may be dispositive. On the other hand, for a legacy applicant with the same stats and ECs at a large public high school that has a not-especially-close relationship with the legacy college, whose family is lower-middle-class, doesn’t donate significantly and isn’t otherwise involved, I think the legacy is at best a feather on the scale. The card is most valuable to a skilled player who’s already holding a good hand.
What this means, though, is that one can’t make one-size-fits-all statements about the value of the legacy preference, in particular because it’s impossible to determine how much the legacy itself (as distinct from the rest of her application, the advantages of her elite private school, the generosity of her family and the fact that she’s a full payer) contributed to Applicant #1’s outcome.