Some really interesting points Sugar78 and wonkit. I'd caveat everything by saying "it depends", and that there are two main perspectives on this:
The first is audience orientated and is simply representation in story-telling, which is important to everyone, and especially minority groups, and that comes from having writers and other creatives who are aware of, and preferably have a good understanding of life for the relevant group. Realistically, that means people with the relevant lived experience, but practically, it could be an experienced writer putting the effort into really good research and getting the right people involved.
The second is actor orientated, and asks whether or not actors from minority groups, including those with disabilities, are getting a fair deal and the same opportunities as their able-bodied counter-parts. I'd argue that if the character is a wheelchair user for the duration of the story, then there probably are enough actors who routinely use wheelchairs to try to find one that's capable of the part before widening the net to include able-bodied actors pretending. I'd also argue that it can be quite hard to pretend to be a wheelchair user if you've not lived it for a period of time. On the other hand, many stories involving disability include changes to the level of disability. The story of Stephen Hawking required an actor who could be able bodied at the start of the film, then degenerate as the years passed.
A problem is that the desire by producers to use the biggest name they can convince to take part that's capable of a role gives the more experienced able-bodied actors a head start, and if your priority is for the general public to see these stories, then you'll be happy to see a big star like The Rock in a big budget film playing a man with a prosthetic, rather than the indie equivalent with an unknown.
An interesting point is how many roles really need the character to be able-bodied? Could minor adjustments make the part suitable for an actor with a disability? There's been a trend recently to even up gender balance in film-making, so when translating male dominated books to the screen, questions are asked about whether any of the characters could be replaced by a female equivalent. Sometimes it's a straight swap, but you can often maintain the integrity of the plot with a few adjustments to make your new female character realistic. I don't see why you couldn't do the equivalent to create more roles for disabled actors. By that I mean that there may be times when the disability is entirely irrelevant and doesn't get a mention, but realistically that sort of thing would be mentioned every now and then, so it doesn't need to be ignored when creating a character who happens to have a disability.
The example of a gender switch that's coming to my mind was the TV adaptation of The Night Manager from a couple of years ago. The character played by Olivia Coleman was originally written as a man in the books. There was no reason why that character had to be a man, so they switched it, and Olivia was pregnant at the time, so they made that a feature of her character with appropriate references.