Could you give me some more information about why this is a bad idea (or direct me to a resource explaining this)? If the problem is that the server uses a lot of system resources, I am using a Mac Pro 2.8 ghz 8-core machine, with 16 GB of RAM. It should hopefully be able to handle the load.
The reason that you shouldn't so it has nothing to do with its resource requirements. The reason is that servers are not meant to be used as clients. Their job is to allow services to run without interruptions as stable as possible. The only time you should be doing anything on them is to utilize one of their specific functions that cannot be accomplished on a client desktop. There is a reason that there is a division - both have different purposes that are not intended to cross over. Server Operating systems are intended to be their own system to complement client OS. Its not something that you upgrade or replace. A server OS is inteded to do a limited set of functions that require minimal human interactions.
Again, do not look at Snow Leopard Server as an "upgrade" like how you see Windows Home premium-> Ultimate. That's not how it works - they are different entities that have different purposes.
If you need a server solution, it should be its own system - not something that you work on general pupose.
It is also good to know that you are using the server as your primary desktop and aren't having problems. That is my plan as well.
I strongly recommend against it. If you have no need for any of the server tools, than you clearly do not need the server OS - it will be a waste. If you do need it, then I would highly recommend keeping it on a dedicated system - expecially if you want to ensure that your services remain up and running.
Apple discourages it for a reason - its not good practice from an administrative perspective and has the tendency to have problems down the line. My perspective always has been that servers are dedicated systems, not general purpose. If you are doing general purpose computing, a server OS is a waste and will only server to frustrate you more than it's worth.
Out of curiosity, what are your intentions for running a server OS?