In RFC 2440-bis5:5.13, it says:
There is a corresponding feature in the features signature subpacket
that denotes that an implementation can properly use this packet
type. An implementation SHOULD NOT use this packet when encrypting
to a recipient that does not state it can use this packet, and
SHOULD prefer this to older Symmetrically Encrypted Data Packet when
possible.
This doesn't, however, give any indication of what to do when using pure
symmetric encryption. What is the preferred behavior when symmetrically
encrypting a file using AES? Should an OpenPGP implementation use the MDC
by default?
--Len.