Really thinking you are reading too much into contrived conflict. Without something for Ranma to fight what point is there to having him/her there?
Problem is that it´s usually poorly and blatantly contrived. So even if some logic background was properly created, it still manage to feel contrived.
For example, if the one Knight of Ranko is referring to is the one once started to read, well lets just say that it went weird with extreme prejudice and then took a turn towards stupidly bad and i just shook my head and quit reading because no matter what, noone could have rescued that story back to something decent.
The senshi in general and Setsuna in particular becoming ever more and more inexplicably antagonistic and stupid.
I´m not sure i´ve seen the setup work out well more than maybe once or twice without the source of conflict being something more or less accidental or based on misunderstandings(and most of those felt badly contrived as well).
And Ranma doesn´t have to have someone to fight to make sense there, there´s one that puts him there as married with one of the senshi, and the whole thing is played mostly for comedy with just some backstory of troublesome politics. Hmm, i cant even remember which one of the senshi he was with, was one of the first fanfics i read, so it´s a while ago.