Packet id has been added to nearly all packets as a resolution to isse 619: https://issues.oasis-open.org/browse/MQTT-619. The only (unicast) packets that do not now have a packet id are DISCONNECT and AUTH.

When DISCONNECT is sent by a client, it expects DISCONNECT as an acknowledgement to signal the Virtual Connection has ended. AUTH packets may be sent by the server as a response to the CONNECT or AUTH packets. So both of these can be responses to another packet, which according to the logic in issue 619, I believe means they need a packet id.