Help: validating users with pdc

[Juergen Sandner]

I have told my samba server to use my pdc for password validation.
(security = server). This works just great, but:

Why does the samba server also check the password in the unix
/etc/passwd. In my opinion it only needs the user-id and
group membership, the password is already checked on the pdc.

Because the samba server is running as root, it should be no
problem to use the uid of the specified user, without going through
the unix password authentication.
ok, i would still have to take care to have all the users I need in the
/etc/passwd, but i wouldn't have to care about passwords, that are
changed on the pdc via the nt clients.

So this is my question:
is there a way to stop samba from checking the password in the
/etc/passwd?
is there a major security problem, that I didn't see?

Thank you for your help