Actually, I wasn't really thinking so much about disk space or CPU so much as RAM. I don't know of any gui that doesn't have a significant memory hit. Memory and CPU are pretty critical on servers. As for configuring the server, is there a way to export the desktop? (XDM?) I can't recall the last time I actually sat down at the console of one of our servers, so unless you have specific tools that you could call and export the display on, the pretty GUI won't do much good.
Granted, being essentially BSD, the servers should be pretty solid, I'd still think they'd need a different OS load for servers. Can you boot to runlevel 2 or 3 on OSX?
Aqua mainly uses the GPUs RAM, and any system RAM it does use when idle probably gets swapped-out to VM.
As for configuring the server, is there a way to export the desktop? (XDM?)
I'm not sure. Obviously you can put X11 on it and go that way. Aside from that, Apple does have their Remote Desktop product (which, BTW, rocks).
Can you boot to runlevel 2 or 3 on OSX?
I don't know about that one. But if you need to get into a mostly-dead system for troubleshooting, Apple uses EFI with lights-out administration. Aside from that, if it's a feature of FreeBSD, it's probably a feature of OS X.
OS X boot process with info about runlevels: http://www.kernelthread.com/mac/osx/arch_startup.html
In a rack of multiple xServes, all of them can be administered by the GUI on just one... or from a workstation connected to the servers... and that one will drop the GUI back to idle and non-RAM usage when the administrator signs off.