The first five tags used on a post show up in the /tagged/ Tumblr page, a much better chronological listing of that tag’s usage than /search/, which just puts mostly everything in some random Tumblr order. I don’t necessarily want to put posts in the main tags but do want them in mine.
#replies has become a catchall standard tag so nobody uses it for real content and #femslash related stuff is pretty much only mine so again there’s nobody checking there who I’d want to be careful of spamming.