exportData field in ExportedPortlet Structure

I might have asked this before, but could someone clarify why exportData 
field is required in ExportedPortlet structure.

If the producer does not support export by value, and the consumer sends 
an exportPortlet request with exportByValue=false, the Producer should 
be allowed to return the handles of exported portlets without 
exportData. Any reasons for making this a required field?

Subbu