I think we're all well aware of the vagaries of slot times GWYN. I think most crews would prefer to operate into discretion if possible to get the aircraft home on the last sector. We all know slot times can go forward as well as back. What people are less willing to do is depart main base already knowing they will have to use an amount of discretion for the return leg, then being subject to even more delay on the return leg which would require to use more discretion than you had previously planned on and considered reasonable given the preceeding events. The decision on whether to operate into discretion is not one of commercial expedience or customer convenience, it is one of safety. Nobody will thank you when your discretionary flight goes off the end of the runway because you wanted to save the company some money by not cancelling. I recently had a fairly gruelling 12hr, 3 sector duty in atrocious weather during which we both decided we were unwilling to use discretion for the last sector, even though it would be a ballache not just for the company but us personally. 90 minutes later, approaching midnight, when we found ourselves 90 miles south of track, still heading south, bouncing around the tops of CBs at FL380 and working out whether we could make destination or would have to divert into an unfamiliar airfield at night we were glad not to have an extra 2 hours of tiredness to contend with.