In considering the use of the AvailabiityType instance to specify a single, non-recurring interval of availability, my understanding is that this would be specified using the AvailabilityType.availInterval attribute. In this case, I would have no need to specify any kind of recurrence using AvailabilityType.rRule.
Therefore, I am requesting the committee consider making the rRule attribute optional.
The alternative, I suppose, would be to have an empty string as the value of a mandatory rRule, but this seems less elegant.