Comment 1 for bug 609092

Could you precise what you mean by "installed by default" ?
It certainly isn't installed on Ubuntu Desktop by default, and it's not installed in the default Server install either. It's installed in the "Samba File Server" task in the server installer. It also gets installed when you enable file sharing on the desktop. In both cases that sounds like a sane default behavior.

The reason why it syncs unix passwords when "unix password sync" is off is that libpam-smbpass is an alternative way of syncing Unix and Samba passwords: instead of syncing them at passwd change (which is what "unix password sync" does), libpam-smbpass (if present) syncs them when the user logs in. The way to disable it is to uninstall libpam-smbpass.