RAM Caching - Exploiting RAM to the fullest extent possible
Manually tuning RAM usage is not always as smart as it seems.

Apple have spent quite a lot of time making OS X use memory as efficiently as possible, and so even when you are not using it directly, the OS will be using it as a file cache or for keeping data ready in anticipation of future use.

If you artificially limit the amount of RAM that is available to the OS by fixing some of the RAM as a RAMdisk, then you are not allowing the OS to work as efficiently as it can - and in fact could be constraining the memory available when it needs it, for a RAMdisk that might not be necessary at that time.

The OS tunes RAM usage constantly, and will ensure that it is being used to its maximum potential. Generally, this is the best use of your RAM, even if it doesn't 'look' like it's being used for anything.