I suppose it’s not that it’s unrealistic such a thing might happen, but whether it contributes to the plot or not.
Books and films leave out loads that is perfectly authentic, but nevertheless not crucial to the story. The archetypal example being that people in films are never seen needing the loo - unless there’s going to be some dramatic incident where they get trapped or confronted in there, or discover they’re bleeding or something!
It wouldn’t put me off seeing the film, it’s just that one of the characters having MS is allegedly a bit circumstantial.
I’m reminded of a law exam, years ago. A scenario was painted where it was obvious one of the parties was gay, and you had to give a legal opinion on the circumstances as given. I couldn’t for the life of me work out what bearing anybody’s sexual orientation had on the legal position, as I think it was a traffic accident or something!
But I wasted a lot of time on the question worrying WHY the person’s orientation had been given. Typically, in a law question, there’s very little information you’re not supposed to pick up and use in some way, so I kept wondering: “Why is he gay? WHY is he gay?” Is there some discrimination angle I’m not picking up on?
It turned out it had no bearing at all on the legal implications of what was, after all, a straightforward liability case about a road accident!
It seems the examiners had included it out of political correctness - after all, parties in road accidents aren’t all straight! But whilst it may have been authentic, it didn’t advance the story at all. You absolutely did NOT need to know the sexual orientation of anyone to be able to opine on their liability for negligence, so the information was overkill.