I know nothing about the film industry, but that kind of clause seems to make a lot of sense. Major actors aren't really replacable in the middle of a movie... you'd either need to find a dead-on look-a-like/sound-a-like, cancel the project, re-write the script somehow to limit that characters involvement to what has been shot so far (not always possible), or re-shoot all his/her scenes, none of which sound terribly appealing and all of which sound expensive.

I'm sure you thought of those issues while writing your post, but sometimes I like to think outloud :)