I think the other half to that equation is having the right process for the tool. I agree it would be incredibly frustrating to try to use torch tip cleaners as actual files to file slots. Or even to remove significant material from existing slots. They don't cut quickly and they're fragile and hard to hold. That's why I get the slots very close by saw or knife and then literally just a couple swipes with the "files" to get the width exactly right and round the bottom (versus the squared off bottom a saw will leave). I agree with you that time is valuable, but with the right process there isn't really a time difference. If we make an analogy to sanding - 400 grit paper is a terrible idea if you're starting from a rough saw cut surface. But that doesn't mean 400 grit paper is no good in and of itself. You just have to use it with the right process.
I learned to do it by roughing with a saw or knife then finishing with torch tip cleaners (or a bit of sandpaper wrapped around an appropriate gauge drill bit for larger strings or a bass guitar nut) before I even knew there was such a thing as purpose made nut files and before there were internet forums to discuss such a subject. So that has probably given me a different perspective.
I'm not going to try to advocate that this makes sense for anyone else though, of course, as people have to be comfortable with what they're doing themselves!