RefToMessageId in DR & Ack

Since a DeliveryReceipt element or an Acknowledgement element can be on any
message, do they need their own RefToMessageId elements since the RefToMessageId
in the MessageHeader might be in use by something else?

I am tempted to say that they would never be on a message in which the
RefToMessageId would be different, yet is it really never?

Regards,

David Fischer
Drummond Group