Interesting argument. There is no excuse for using an able bodied actor when a suitable disabled actor could be cast but I think the analogy to "blacking up" is taking it too far.
"Blacking up" was essentially at "best" a lazy parody or caricature of black people. It is now seen for what it is -racist stereotyping. I can't recall a serious film or play where white actors played black characters. More likely black actors played black roles which reflected the social distinctions of the time (e.g. in a role of a domestic).
An able bodied actor playing someone with a disability is them taking a role. There is no mocking or parody. It's not a caricature. Where is line drawn? Does a role requiring the actor to play a character with a mental illness require them to be diagnosed with that mental illness? Should an actor with Downs Syndrome play a character who is confined to a wheelchair with a degenerative disease because no suitable actor with the direct disability can be cast? How closely does the actor's circumstances have to correspond to the disability portrayed in the role?
Acting is exactly that. Actors do not have to be the person they are portraying in real life. If a role calls for an actor to lose a limb in the course of the story then what happens?
I have relatives with disabilities and fully subscribe to the view that they should not be stopped from participating in life just because they have some extra needs. There is no doubt that the casting of disabled actors in both roles portraying disabled people as well as roles of a person doing X that just happens to have a disability is woefully poor. Disabled people are under-represented.
My personal opinion is that to make some blanket statement about this being equivalent to "blacking up" does the underlying argument no favours. Concentrate on helping disabled people getting into the film, TV and theatre industries. Work with studios, directors and producers to breakdown the walls that are there. Show some best of breed disabled actors at work. Educate.
