If buyers know they have to beat the ROFR, they'll offer more.
That's not sufficient unless the rightsholder has (effectively) unlimited resources. And, not even Disney has that.
This is simple economics. At any particular market price, there are a group of willing sellers, and a group of willing buyers. As the market price increases, the sellers grow, and the buyers shrink; and vice versa. This is especially true for a luxury item like DVC, which is elastic in demand. At any given time, there is some price at which the number of buyers is roughly close to the number of sellers, and that's the "market equilibrium." It's influenced by the number of owners who are tired of going to Disney every year, the size of the informed resale purchasing market, and so on.
(Some are surprised by this elasticity. Among the fanatical, it's hard to remember that Disney isn't in the same category as food, clothing, and basic housing. But I digress.)
Disney cannot change that equilibrium price, except as another participant in the market. They *can* set a trigger point higher than that price, and *some* prospective buyers will decide to pay a higher price. But, others will balk, and do something else with their vacation dollars. In other words, the pool of buyers shrinks.
But, the pool of sellers grows. So, more deeds come available on the market. Among those selling are folks who think "I still like DVC, but for that price, I could sell." There are also folks who, for whatever reason, *need* to sell. These are your "motivated sellers". They will try to undercut the market a little bit, because they need to get out from under the deed. Some buyer will take a chance on it---either because the price is worth spending a little time on, even if it is ROFR'd, or because the buyer doesn't know what the trigger price is.
When that happens often enough, it doesn't matter whether Disney ROFRs or not; deeds will sell for less. Disney either needs sufficient resources---and a sufficiently developed direct sales channel---to ROFR and then sell all of them, or they need to let some go through. That reduces the *known* trigger price. And so on.
In effect, Disney acts as another purchaser in the market place, but one that does not have to competitively bid for inventory. But, that purchaser can only "make the market" if they are willing to buy *all* inventory at a particular price. That's rarely true, even for someone with pockets as large as the Mouse's.
Now, if you go back and study carefully the data in those links I provided, I think you will find that, during 2009, prices were dropping fairly consistently, even in the presence of ROFR. ROFR was essentially discontinued in early 2010 everywhere except BCV. So, Alan has it right: prices dropped then ROFR was curtailed. This probably does cause prices to drop a little faster---Disney is no longer a willing buyer in the market place, and that reduces the market equilibrium price further. So, ROFR does have *some* positive pressure on prices, but unless the rights holder is willing to go all the way, it is hardly the silver bullet some make it out to be.
The annual pass discount is nice, too. 
