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.
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.