That is preferred for multiple reasons. Mostly having to do with keeping your life simple, and conveniently also being security best practice.
You do not want any ports exposed to the WAN on your DC to minimize attack vectors. Having that Nethserver only host a DC container is perfectly defensible from that pov.
On the easing your life part: You really do not want to have to reboot a system co-hosting the DC for whatever reason, and you really do not always want to have to wait for off-hours before you reboot.
You have not stated the scope of use (private home server/small business/whatever) so this might apply less, but it is still not a bad idea. There are no resources lost, and I get away with 2GB of memory on our 15ppl company. Diskspace for this scenario is something you can almost ignore … I gave it 50GB but that is an absurd amount of overkill. I have 40TB tho.
So of you can host a server on your virtualization solution running a DC and your other needs, you can run 2 … one with a DC, and one with the rest. It really is not a bad idea to have 7 servers dedicated to one task instead of 1 doing 7, even if it is running on the same machine. The slight hassle with having to do things 7 times (updates for instance) pay off when one of these updates has a special effect, and renders the server less-operable… you will have 6 services up in my scenario … and 7 down in the other.
See image below for our Samba DC hosting Nethserver’s dashboard; as you can see, it needs little.