[Samba] Why warning: "Server's Role (logon server) NOT ADVISED with domain-level security"

[L. A. Walsh]

When using "smbmount" (but not mount.cifs), I get the warning
message:
Server's Role (logon server) NOT ADVISED with domain-level security

This has appeared after I added "security=domain" back into
my smb.conf file (seems I or an upgrade deleted it last December).

Why is it "not advised" to use the same machine for logon server
and domain security?

Is it just an "ommission" that it doesn't warn me with mount.cifs,
or does the warning not apply there?

Thanks,
Linda