I'm already planing some major changes to the account system. Since Activision will take the masterserver down soon, we'll have a problem with the GUIDs
Furthermore you make a good algorithm that looks how good you perform. And that creates a factor that affects you damage on the zombies and the zombies on you. This way you leveling difficulty a bit more out.
The stats are there motive to play. Maybe there should be two groups. one with a standard average kit where they play with. and the other where you start with a bad kit + stats and can grow up to a good kit. First time you join you start in the first group and with !register (seen it in other mods) you can join the second group.
Every time the master server is down, the number of players is lower than normal. Think this is because they don't play with their own account anymore. And so the don't get there stats. With other words there are players that only play because there stats getting record.