Just looking at cerebral palsy, there is a large range of disability and need for mobility devices.
My youngest daughter has cerebral palsy, spastic quad with dystonia. So, for the non-medical people, her whole body is affected and she has both spasticity (stiff and hard move muscles) and unusual muscle movements.
She does have her own lightweight manual wheelchair and her own power wheelchair with specialized seating systems in both.
At one point in time, she used a regular walker, but she had a gait (the way someone walks) study which showed she was using 3-4 times the energy to walk a specific distance than a person without disability of her size and age.
In addition, because of her abnormal gait, we were advised by her Physical Therapist and Orthopedist that she was at high risk of arthritis in her knee, ankle and hip joints.
That is a known complication of CP because of overuse of joints.
We elected at that point to stop the walker because, in addition to those things, she has a high risk of falling because of her gait.
I have read studies and professional magazine articles about the subject, especially in regard to parents wanting their children to walk at all costs.
Standing/weightbearing is important for many reasons, so she uses a strander and a gait trainer to safely be supported during movement. We no longer look at walking as a primary means of movement.
There are many people whose condition allows them to walk during their 'normal' day, but not the 5-6 or more miles at WDW. (again, only talking about CP, although the same applies to other conditions).
I personally know people with CP who have only one side affected or who have weakness in both legs.
They don't own a wheelchair or an
ECV, but do rent one when they go somewhere with much walking as WDW.
I also know some people who can walk a bit (and one who can't) but prefer to use an ECV instead of a power wheelchair, mostly because a power wheelchair is harder to transport.
I am not a therapist. I am an RN who has worked with people with disabilities for many years in Home Care, Schools and a Rehab Hospital.