I've got to say I have noticed it.

On a couple of the flights that I've taken where I've noticed it taking longer, or flights where it's taking insanely long to clear, sometimes I see the Y-up (P/I) fare nearly identitical or maybe $20 more expensive than the cheapest coach fare.

For example, if I see, JFK-SJU H coach fare at $521 but I fare at $541. I think in these cases, revenue management believes that they are highly likely (and I'd agree with them in this case), to sell the instant upgrade fare, and that's why they hold back seats. If I'm a leisure traveler booking an immediate trip to San Juan the next day, I'm almost certainly going to cough up the extra $20 for an upgrade.

The issue I do take is that when they are holding seats, its often times when the cabin is very empty, and thus sometimes causing EXPs to clear with lower level elites-diluting the ability to select ones seat.

It's interesting to notice these patterns though, my philosophy is if AA RM actually sells most of the seats they held back, then they did their job. If they are hardly selling at all (which has been experience on my flights), then perhaps they need to retool their system to not anger elites by holding them back such a long time for no reason.