What if there are no disabled actors available and capable of playing those parts?
One of the problems with allowing (say) men to play female parts and white people to play black parts is that it makes it easier for directors to say just that - to be naturally set against hiring a person who would naturally fit the role, make a cursory search, claim that they couldn't find any black people or women suitable to fit it and hire a white person or a man. By not allowing this to happen, you at least force them to look hard.
That said, I can now see an immediate problem - does this mean that the only actors who can play Richard III are those with spinal deformities? Perhaps the question should better be put positively: ceteris paribus, there is something innately detracting about making a non-disabled actor ape a disability, and something innately good about giving that role to somebody who does not have to pretend - especially when the disability in question (such as a mental illness like Tourette's, for example) might lead people to think that the non-disabled actor was mocking it.