First, promotions do not always save you money. My previous clients booked trips before the annual ticket increase and before the parking fees were implemented; they would have been charged $350 if they switched to/applied the promotion.
Also, many times you are required to have a certain length of stay park hoppers/park hopper plus park passes. Thus, promotions can sometimes lead to less flexibility overall.
Another example is if you have booked an Animal Kingdom Savanna view room, you may need to downgrade rooms if the standard rooms are where are the savings are at. Or, the Polynesian you might have to upgrade from a standard room to a preferred room–under this scenario you may be getting a “free” upgrade, not a discount.