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).