You need encrypted time sync when you don't have a local trusted source, and to reduce your susceptibility to to pseudo MITM attacks (though, having just said that, you're right - UTC isn't a secret, so an attacker could just use that. However, and I'm not sure the answer, the key may be in how much information is leaked to the NTP server on current clock drift state. That being said, I'm fairly sure Kerberos etc don't require millisecond level time sync...)
You are confusing authentication and encryption. Authenticated time protects against MITM attacks. You can obtain authenticated time synch from many national labs, such as NIST[1] or CRC[2].
Well, my thought was that you might be able to determine local clock drift by snooping. But as then mentioned, that's of near zero use, I suspect (as anything that uses time sync for authentication or otherwise is not likely to be that tightly bound to accuracy).
