I am confused why people who don't read or hate villainess stories, and therefor presumably would use it, are arguing.
It's a tag, you include it, exclude it or ignore it. Even if you just search the word 'villainess' here there are 54 results that turn up, I'm pretty sure that's more than some of the other existing tags already and wouldn't catch all the villainess stories.
btw, I do like reading the villainess stories, shoujo, josei and senien (some of them), I am unsure if I've read a shounen villainess story though. It'd be nice to be able to search for them easily, especially the ones that don't turn up when you search 'villainess', I have a bad habit of forgetting to follow some stories and being unable to find them ever again because I can't remember their title and they don't turn up in the searches I'd expect them too.