If anyone IS invited for tea and biccies for NOT using discretion, they should be writing to their BALPA rep, their FOI Inspector, CHIRP, and their MP.
If a particular route regularly incurs delays, then it will have to be rescheduled. Airlines are not entitled always to plan for the best possible result with Handling delays, ATC slots, late pax, baggage mix-ups, poor routing, etc.
If anyone encounters significant delays down-route, then the Captain, taking all factors into account, particularly the fatigue states of the crew, but mostly SAFETY, should make a decision on whether or not to exercise his discretion to extend the flight duty period.
It is not available to the Airline to question such a decision, unless that be to suggest that perhaps he decided to extend when he should not have done so. End of story.