Wouldn't it be way easier to just encrypt the partition? Ubuntu for example has an easy-to-use feature for that. The idea of email encryption is not least to avoid interception in the clear.
You should do both. The solution described in the article provides several extra levels of defense against different types of attack that full disk encryption doesn't provide. If you actually read it, you'll know this...
The problem with filesystem-level encryption is that it allows the mail server to decrypt the messages. With PGP, the server is only able to encrypt messages; messages can be decrypted only by clients which have the private key.