Running OS X Server on main machine I'm considering installing the OS X Server app on my main, day-to-day machine, in order to run Xcode Server and get a GUI for the VPN configuration (I'm already using the VPN with a manually configured LaunchDaemon).

Are there any reasons I shouldn't do this (such as security or performance issues)? I don't have another Mac that can run Mavericks at the moment, and would turn off all of the services I won't be using.
Generally, installing OS X Server on a main machine is not recommended, but if you have no choice, it should still be fine. There are some things to note, of course…

PostgreSQL has a massive impact on boot up times and shutdown times. PostgreSQL is used by many of the services in OS X Server and has meant that my iMac now takes well over 3 minutes to shut down. However, overall performance in the system is generally unaffected, providing you don't enable all the services or be running computationally-intensive tasks. For example, it might seem obvious, but running a build with an Xcode server CI bot will kill performance as it uses all the CPU available, leaving none for the current user. This is opposite to many services in OS X which will limit CPU usage to leave enough for a good user experience.

With regard to security, providing you are aware of what you are enabling, you shouldn't have any problem with this. The largest thing to note is that OS X Server will, by default, enable SSH access, which you may wish to disable. Everything else is by default disabled.