There are times when the plot changes and for example, in a romance, characters may switch from using surnames to first names. I've seen many low quality subtitles where the character will use another character's last name, but the translation will use the first name. While it does require some knowledge of Japanese (or other Asian cultures), it does contribute to the overall consumption of the anime, and only distracts from the attempted transition from Japanese culture to Western culture as translated.
I mean, that's important, but the other nuances I mentioned can change in just the same way. An adept translator can capture these things without resorting to lots of untranslated Japanese (although whether the people doing fansubs are adept is a different question... often there are quite obvious misunderstandings of what was said in the fansubs I've seen).
