In my original plan, you wouldn't have seen Yuffie's 15 minute trips out. You would have just seen Rikku's trips and then everything else would be left to your imagination. But I thought that was probably unfair.
I wanted to make it clear that unlike Rikku, Yuffie had no guilt in using the materia on unsuspecting people. I thought that made Rikku's parts of the story coming up more funny. She's in a more and more unreasonable situation.