One of the prime reasons for a GTD system is to facilitate dealing with the unexpected and the unplanned. In those instances, you must have a complete inventory of your commitments, in order to be able to assess the relative value of the new input/circumstance, and allocate your focus appropriately. Probably 95% of my usage of my own system is to allow me to feel comfortable with a change of plans.
You want to keep your backlog of unprocessed stuff at a minimum, and an inventory of all possible actions close at hand, so you can optimally deal with surprise.
Link:
Handling “emergencies” with GTD
