I don't understand this reticence about POSIX compliance. When would you need your shell to be POSIX compliant? I've been using fish for ten years and I've never had an issue, even with scripting. If I need to run a script, it always has bash in the shebang line, so it runs normally.

Am I missing something useful?

Other people's scripts in projects where you can't or don't want to edit the scripts to add a shebang line.