The machine I keep my Calibre library on and run my web server on is a NAS box that is not capable of running Calibre. Leaving a PC switched on just to run Calibre Server is not appropriate.
I have already agree with Kovid that the long term aim is to make the facility available as part of the Calibre suite. The intention is to be able to run the facility in both free-standing mode, and also as part of the Calibre server. However in the short term the free-standing facility is likely to come first as I can work on that independently of Kovid's work elsehere in Calibre and leave the integration into Calibre server as a second stage.