Its not really the "one user at a time" that is the question.
The server comes with GUI to let you do DHCP and DNS. It has a mail server. It has Apache set up. There is a software update service in it (so you don't go back to the Apple site but to the local site for updates). And the list goes on.
I have 10.4 server. I don't think I'm going to upgrade to 10.5. I sorta don't like it. I'm an old fart Unix guy that has replaced most of what comes pre-installed with the latest up to date versions (10.4 came with Apache 1.2?). I end up fighting with the GUI more than it helps me.
There is very little in the server that is not from open source. So, if you need this or that, and are competent at compiling and installing the software yourself, these is not much point. And the $500 price tag gags me.
But, if you need server facilities and want as nice of a GUI as I've seen, then its a good choice.
HTH