Companies sometimes have to resort to rostering tricks like Level 2 variations to make them work (essentially using an hour of discretion at the rostering stage).
And there was me thinking that rostering discretionary time was a hanging offence.
One of the many good reasons it should be is, surely, that it is "discretionary" time, available for a Captain to use within severe limitations in certain unforeseen and unforeseeable circumstances. If it's rostered it ceases to be discretionary, and the Captain is left with less (or no) further discretion to manage the unforeseen.