smbclient sends information to stderr even when the debug level is 0 (from smbclient(1) "At level 0, only critical errors and serious warnings will be logged.")
$ smbclient -d0 //localhost/test 2>err.log
smb: \> quit
$ cat err.log
Domain=[WORKGROUP] OS=[Unix] Server=[Samba 3.0.28a]
IMHO the Domain=... should go to stdout instead of stderr because there is no error.
I agree this is a bug. However, fixing it will probably break a lot of scripts out there, so we'd rather keep it the way it is for legacy reasons.
I'll make sure we'll fix this for Samba4.