This bug is an old one in the Debian BTS. I have no precise opinion on the 
suggestion as this part of the code is not something I use myself. 
 
The bug submitter proposed a patch which does not apply completely cleanly on 
current sources, but seens easy to adapt. 
 
I personnally classify this as a feature request. 
 
The bug submitter said: 
The auth component of libpam-smbpass contains an option, 'migrate', 
which adds users who authenticated in a prior module to the smbpasswd 
file, in order to ease migration to smbpasswd. However, this means that 
new users must log in at least once to be added to the smbpasswd file, 
making it still partially difficult to keep the passwd and smbpasswd 
file synchronized. 
 
This patch adds migrate functionality to the password component of 
libpam_smbpass. This means that, if the migrate option is set, 
libpam_smbpass will add the username and new password to the smbpasswd 
file if they are not already in it; if they are, it will change their 
smbpasswd. 
 
If migrate is not enabled, the module will function as before. If 
migrate is enabled, but the user is already in the smbpasswd file, it 
will function as before. 
 
Though there is the slight security risk that any user's password will 
be blindly changed by this module, and the risk that new users will be 
happily added by this module, neither is possible if the module is used 
properly. If the modules is preceded by pam_unix in requisite mode, 
pam_smbpass will not be excecuted if pam_unix fails- which it does in 
both cases mentioned above: an unknown user, and an invalid old 
password.  
 
This is a security issue because it enables keeping the two different 
password databases synchronized, allowing for better system management. 
If a password is changed from within Samba, enabling the 'pam password 
change' option in smb.conf will keep the UNIX password database current. 
If a password is changed via PAM, pam_smbpass will ensure that smbpasswd 
is updated. If a new user is added, the first time their password is 
changed, when their password is first set, probably by adduser, they 
will be automaticly added to smbpasswd. 
 
Care must be taken to remove users from smbpasswd when they are removed 
from passwd, but this is not new to pam_smbpass or this module.