I wouldn't like to rely on a RISC OS system to perform server tasks whilst I have no physical access to the box. There is software which can make remote administration easier (VNC, remote shells) but a software error can cause unrecoverable problems. In situations that require a server, I would use an OS that is tuned to that end eg. Linux.
As a front end, RISC OS excels in every department. It's responsive, has a great UI and is generally a pleasure to use. Linux is a great OS for server software because it has a reasonable threading implementation and is more stable when software misbehaves.
This is not to say that either of the OS above could perform the other task, but it would be unwise to use RISC OS in a server environment in my opinion.
I don't want RISC OS to be an all rounder, I want it to excel in the area it's good at - the client side.