by SpaceKnight of Chaos » Fri May 21, 2010 4:01 pm
I think it might be so common in fanfics because, while the two do speak to each other on occasion in the actual series, there doesn't seem to be much evidence that it's something that happens very frequently, or that it's anything more than idle small talk. Kasumi is kind and polite to Ranma, because Ranma is a guest and she is kind and polite to everyone, and Ranma reciprocates because it's in his nature to treat people the way that they treat him. They don't really seem to share much common interest, though, and so usually they just pass each other by while living out their own lives. Kasumi mostly is seen running errands or going out to visit friends (the latter especially early in the series), while Ranma has a... shall we say, more active life?
Ranma spends most of his time either alone or with Akane. While part of it is because they are engaged and so Genma and the other Tendos probably do force them to spend time with each other, they also have grounds to be with each other. They may be uncertain of their feelings for each other, but they do consider each other friends, they attend the same class and school, they share many of the same problems, and share at least some basic interests and hobbies. In short, Ranma has many reasons to spend time around Akane and interacting with her, while canonically he doesn't have any real cause to spend time hanging out with Kasumi (and she doesn't have any real cause to spend time with him, either).
The scene you specifically mention was most likely Kasumi offering to sew up Ranma's shirt, it being the sort of thing she does, and Ranma using it as an opportunity to ask why Akane was so worked up over the whole Romeo and Juliet thing; as her sister and the stand-in mother of the household, Kasumi is almost certain to know the answer, and talking to her is much easier and more pleasant than talking with Nabiki. Then again, so is driving nails into his eyes.
Water, water, everywhere, and all was cursed and black!
Drowned ones cast bad spell and out come pig, girl, duck, panda!
Swirl, swirl, slithery pond, and join with magic spring!
Swirl, swirl, dirty pond, and rid the cursed sting!
Here my prayer, I beg you please!
Now turn these curses BACK!
http://www.issendai.com/rpgs/japanese-boys-names.shtml
http://www.issendai.com/rpgs/japanese-girls-names.shtml