There's like one or two use cases where encrypting email could work. The best case I've come across--Bugzilla has the ability to let the user upload a public key to encrypt emails for updates to non-public bugs. It's not a big use case--pretty much the intersection of "must use email" and "can establish identity out of band," which does not describe most communication that uses email. (As tptacek notes in a sibling comment, you pretty much have to limit this to one-and-done stuff too, not anything that's going to be in an ongoing discussion, because leaks via unencrypted replies are basically guaranteed).

Your mail either needs to be encrypted reliably against real adversaries or it doesn't. A private emailing circle doesn't change that. If the idea here is, a private group of friends can just agree never to put anything in their subjects, or to accidentally send unencrypted replies, I'll just say I ran just such a private circle at Matasano, where we used encrypted mail to communicate about security assessment projects, and unencrypted replies happened.

Even my doctor's office and local government agencies support PGP encrypted emails, and refuse to send personal data via unencrypted email, but tech nerds still claim no one can use it?