Why is this extra completion acceptable? On Debian I install the package adb and get the udev rules with it. I just need to connect a device and run adb commands.
Because nixos manages your system and nixpkgs only contains packages. Packages cannot do anything outside their installed location, so can't touch udev or groups to make the changes. There will be other apps split in the same way.
The benefit is that just adding some package will never break your own configuration.
The benefit is that just adding some package will never break your own configuration.