Perfect forward secrecy requires two-way real-time communication, in order to construct a session key that can't be computed from just the private keys and the encrypted message. Therefore the way that PGP's lack of perfect forward secrecy is a feature is that it allows an encrypted message to be generated in a way that doesn't require two-way real-time communication, and can therefore be sent by email.

The trade-off is that you then don't have perfect forward secrecy.