1) it is now much easier to use with Mac mail. The GUI is simple to use once configured. Gpgtools.org has done a great job.
2) when you ‘sign’ email it appears visibly different than otherwise to the recipient and can be a good anti-phishing indicator.
3) I agree with Moxie that GPG encrypted mail is rare. S/MIME with its support from Microsoft, seems to be winning in that space.
* Add an authentiction subkey and you now have a drop-in-replacement for ssh keys.
* Password managers like "Password Store" use a git repository full of gpg encrypted files as a backend allowing for a secure password management solution that only decrypts the credential you need vs decrypting -all- of them at once like most popular tools.
* If you make use of the solid smartcard support you can even avoid your keys ever being exposed to system memory where malware can steal them.
S/MIME and X.509 generally are used in enterprise scenarios where people want one corporate Certification Authority and thus keeping things simple (the underlying encoding is far from simple but that's another topic).
[0]: https://www.comodo.com/home/email-security/free-email-certif...