Yeah, I don't tend to use "metamour" much either, for basically the same reason. MC and TGIB are friends in their own right, and will hopefully be roommates in the future. That's enough labels for me. I can see thinking the word is a little silly, but hating it just seemed a bit strong. And it CAN be a nice shortcut, instead of "my boyfriend's other girlfriend" if it's a situation where I don't know her or don't have my own relationship with her.
Pan Female, Hinge in a V between my mono (straight) husband, Monochrome and my poly (pan) partner, ThatGuyInBlack