I'm curious how the system would work otherwise.
If all annual passholders were allowed to use MyMagicExperience and reserve FastPassPluss return times throughout the validity period of their passes, what's to stop the entire population of annual passholders from reserving spots on Midway Mania for every single non-blackout day of the year, just in case they might want to go? The same is essentially true of people who buy single-day passes as well. Additional constraints and mechanisms have got to be put in place for park guests whose time of visit cannot be keyed to a resort stay.