My biggest motivator for using encrypted email isn’t privacy (or paranoia); it’s politics. The TSA and other agencies consider any attempt to retain privacy strong evidence of terrorist activities. By using encryption—particularly for completely mundane correspondence—you strengthen the culture of privacy among honest citizens. (I’m not suggesting that the case for privacy is entirely clear-cut and won’t dwell on the complexities of the arguments here. Suffice it to say that I believe protection from unreasonable search and seizure is one of the the things the US Constitution did get right.)
If Apple really does consider itself in competition with Google, then privacy tools are weapons Google can’t defend against. Gmail loses all value to Google if all its messages are encrypted such that only the sender and receiver can read them. Apple Mail loses nothing if encrypted mail becomes the norm…in fact pervasive use of certificates, keychains, and encryption all increase people’s reliance on personal computing devices at the expense of stateless web interfaces.