It is recommended that you take a backup of the "data" directory once
in a while in case of system crashes, data corruption, or whatever.

It is VERY important to note that you should only take complete snapshots
of the data directory and only restore complete snapshots of the data
directory.  Restoring individual files from backups can cause problems.
Some of the known problems with partial restores are:

 * In games with MOB_ACCESS enabled, mobility will be extremely out of
   sync if you do not do a complete data directory restore.

 * Land units, ships and planes may think they are loaded onto ships
   or land units that do not exist, or have changed owner (been sunk
   or destroyed and then rebuilt) since the last backup.