Simple economics says the more rooms that are booked the less discount, if any, will be offered, as they will have less poeple to attract.
If you are flexable and are waiting for a disount wait to book, if you want something specific and will book it anyway book the room, but remembe,r if that categy of room is sold out when discounts are released it wil not be able to have the discount applied.
Think of it as a game of bluff, or poker, if Disney do not have the bookings they do not know if you will book so may offer more to get you to visit, if they know you want to go, even with miock bkkings, then they know they can offer less.
Also, the nore people they can offer the dicount to lthe less deepo it will be, for example, I would guess the only reaosn that there is a slightly bettwe disocunt for AP holders compared to the general code is so that the AP holders are not totally alinananted, after al the $5 a nigherr is such a small marginal cost, and these are more likey to be adidiotnal vists creating a much higher marginal income.
So, in short, if you want good discounts, as a gorup, those waiting for them should wait to book the rooms and not make the bookings in advance. Travel agents may not like this, afterall, they may not get the booking, but this is the way that the system can be played, if enough people hold oiut.