When your system is completely save and trustworth you do not need additional encryption. But you can never say that your systems are secure. For example if one of your servers gets hacked your entire security is compromised. With this in mind I would still use additional crypto.

When your systems are completely save and trustworth you would not need additional encryption. But can you be sure that your systems realy are that save? For example if one of your servers gets hacked or if (as makerofthings7-c-lamont pointed out) your backups are compromised your entire security is broken.

When you use something like PGP or S/MIME an attacker would have to steal every private key of any user he wants to wiretap. This may mean that the attacker needs to hack a dozen computers instead of one server to get the same amount of information.