I don't have PM available on all channels and there's the advantage.
You don't need PM for PGP. You can post your encrypted messages publicly. No one else could even tell who the message is for, let alone the contents.
If i post a pubkey on more than one public channel, then there's a traceable connection.
It is trivial to create as many PGP keys as you want.
Sending sensitive information unencrypted to an email address, even a disposable one, is not secure.