Another thing to consider, is that any pilot with his head screwed on correctly doesn't depart, or sign on for duty, knowing he is going in to discretion.
Discretion is for unforeseen circumstances once you have pushed back, or perhaps, if you take the piss, once you've singed on.
However, this is not how the airlines see it. They expect it from Captains from bottom feeder airlines, on a daily basis. This is why tickets are cheap and margins are narrow.
At the end of the day, it's up to the Captain to be professional, once already dispatched! And also take the fatigue levels of all of her or his crew into account.
Safety, and all that.