^The reason there are two tags in the first place is that they allow us to differentiate between the ice fairy Cirno (tag:Cirno) and yukkuri Cirno (tag:Cirnoff). It helps users find things in the database much easier. Or would you prefer a single tag for both vampire and yukkuri Remilia, as well as the other characters?
when you say Cirnoff = yukkuri cirno it makes more sense to keep the tags separate. I'm pretty sure I wasn't the only person who didn't know that and thought cirnoff was extra and redundant.
Lets presume that people searchin on this website search mostly for yukkuris. And when someone new uses one-word tag in most cases he meant yukkuri of chosen type. So, to filter actual characters with one-word name from this searches we should tag them as character:nazrin and character:chen or as youkai_nazrin and shikigami_chen
I don't know. If there were maybe 40 pages of chen and you didn't want to bother skimming through non-yukkuri chen that would be one thing. Skimming an image and saying to yourself "ok, thats normal chen" is probably better then having to go to the tag section to find out why chen isn't finding normal chen in the other case.