At this point, we can't think of any "do not perform" use-cases that wouldn't be better handled by either using one of the more formal 'authorization' requests (MedicaitonRequest, ServiceRequest, etc.) or better be captured as part of a higher level requst/Task describing something to be done.  If someone comes forward with a concrete use-case where doNotPerform on Task is appropriate, we'll happily re-evaluate this decision.