Because XP works, I can leave my system powered on for months.
As long as a Windows update doesn't force a reboot.
That is the main thing that causes me trouble, and forces me to have a coffee break.

I was wondering how Windows 8 server would work ...
Windows 8 in the PC edition supports the "legacy" interface of windows with menus and title bars and status bars. So real applications will still work on a traditional desktop.
But if you have an ARM tablet, then Windows RT locks you into the Metro UI.