True, the usecases overlap somewhat, but they're still different.
I wouldn't want to install n-m (and the 30 libraries it depends on)
in an initramfs, for instance.
Unless I am mistaken, I believe systemd-networkd would be a lot better suited the n-m on servers. Configuration is stored in plain text files, can be recorded in etckeeper/git, etc.
I have nothing against either solution, but trying to solve all problems with one tool isn't going to work, I think.