And why not? Like I said, they don't audition for every role. So why ask another actress to play a minor character just as well, but who doesn't have the same appeal to fans?
The thing is, this "gimmick" is not bad. No one would criticize the movie for it, and it's a plus for some people. So I don't get why it would take away from the quality of the movie.
Not to mention, for the greedy minds, that she probably wouldn't cost so much as another actress, considering they could use more familiar faces for supporting roles.