Discretion has always been the Captain's decision
Hypothetical question time:
You're on sector 3 of 4, and you've been carrying a 20 minute delay after two sectors. Your max FDP gives only 30mins breathing space from your (legally) rostered duty.
You called crewing after sector 2 to tell them that you were running close to max FDP and did not consider discretion to be appropriate due to being on your 3rd early start. They said you should be all right and there was no standby cover available. Ops tell you there is no slot for your last sector so you should be OK.
On arrival downroute you find you have a slot delay of 45mins. You are now committed to going into discretion unless you offload the crew and wait for a new crew to be flown in from the UK which will hugely disrupt the schedule, and you and your crew.
I would say you choose discretion every time. And I would argue that it was purely your decision - more like you have been put into a position where the use of discretion is inevitable. Although technically you
could have refused to go into discretion, it was too tightly rostered to make it much more than a moot point.
Sort these rosters out eJ! Discretion is a "get out of jail" card for exceptional circumstances, not just a means of getting yourselves home after a pathetic rostered duty.