When you combine the fraction of "hooked" admits with the number of seats affected by affirmative action, you get something between 35%-43% (depending on if you discount hooked admits that were extremely qualified anyway). I think this is an interesting way to frame it. Left to its own devices, Harvard would only devote around 60% of its undergraduate program to simply educating very very bright students. The other 40% is/was for les vieux riches, athletes, generally connected kids, and racial diversity.

While I'm sure it varies by school, I suspect you will find a similar dynamic at many other elite private schools. The undergraduate program is going to be much less important to a top-tier research university, and consequently the admissions board can go nuts with other priorities. When even the runners-up (on a merit basis) are quite strong you can go quite far indeed before anyone would notice a slip in standards.