Message encryption (via CRES or other methods like PGP and S/MIME), is different from encrypted message transmission via TLS. The former encrypts the message from the origin all the way to the recipient's eyeballs. The latter only encrypts the message while it is in flight on one SMTP connection. The message is not protected on any other SMTP hops it may encounter (for example, if the destination mailbox is forwarded), nor is it protected when it is at rest in mail queues or the recipient's mailbox.
It is dangerous to assume that TLS provides message security. It does not. Even if the entire Internet switched to encrypted SMTP today, it still wouldn't, since messages would still be in the clear while at rest. If the content you want to protect is sensitive enough to warrant encryption via CRES, then don't bother worrying about TLS.
So what is TLS good for? It's very useful for encrypting passwords on authenticated SMTP connections. It can also thwart traffic analysis, since it encrypts the message envelope (which message encryption does not do). But the latter is only useful when you use TLS on the connections that the eavesdropper is likely to intercept.
