binfmt_misc is not used on normal installs, unless Java, Mono or WINE are installed. Hence I don't think there is any need to compile binfmt_misc into the kernel by default.

In a systemd world the binfmt_misc is actually installed as autofs fs so that the fs is actually just backed and the kmod is only loaded if actually somebody uses binfmt_misc.

Given that binfmt_misc is tiny this doesn't matter too much, but I think it might be oe of the things where making it a loadable module is actually a good thing.