Of course is the whole crew a factor when it comes to the discretion and the Captain will judge whether it should be applied or not.
The Captains discretion is however a valuable tool to provide the crew with some extra flexibility in case of unforeseen circumstances. Now, JAR actually regulates the usage of unforeseen circumstances. The Captain is required to file a report whenever he exceeds flight time limitations by more than one hour and the airline is required to keep track of all extensons of flight time limitations. If flight time limitation is busted on more than 25% of the flights on any given route during a certain timeframe (3 months if I remember correctly) the company is required to revisit the route/crew scheduling. Therefore your argument that this practice is nothing else than compulsory overtime does not hold water.