By design SME is implementing general accepted security practices for file sharing. Users are required password, quotas can be set up, permisions are assigned based on the group membership, guests are not allowed to use the server.
In order to use it you just need to create accounts for your users, and on Windows 9x computers change primary logon to MS Network instead of logon to windows. Use the same name/password in windows as existing users on SME.
If your requirements are different, (home network, very few trusted users, no guests etc..) you may skip samba authentication.
Samba configured only for internal (LAN) interface, so loosing security for samba will not affect you Internet security.