Just because *you* think that tags are unneccessary doesn't mean that 
others share the same belief. Myself, I much prefer to know 
beforehand if the fanfic I'm reading is going to take a darker turn 
or not, or if it's a lemon, or anything like that. How about that 
"Kagato's Revenge" fanfic, eh? Doesn't that one just scream for the 
[Pre-teen RAPE] tag? How about Aikan Muyo! (hope I spelled that 
right), a very good fic that deserves a [LEMON] tag, and shouldn't be 
ashamed of it - it's a well-written lemon. Or the one that started 
this, which deserves a [DARK] tag, and again should not be ashamed of 
it - it's well written, and good.

And just how do you define just what is "Dark", much less the various shades of darkness? 
These kinds of tags sound good on paper, but they don't really tell you anything in reality. 
I'm willing to bet my defination of dark isn't quite the same as yours. I'm also willing to bet 
that the folks who don't write and read Ranma fanfics don't exactly have the same viewpoint 
of what consitutes darkness that the folks that do read and write Ranma fanfics.

Different strokes for different folks....