I was operating one of the LGW trips a few years back when one of our colleagues became sick down route so there was a lot of shuffling around between the various pilots there to cover the flight
The problem was resolved when one of the guys was there on his days off and had filled in the Off Base Notification form and was due to travel home on that day, they called him up and he operated the flight...
Now if the form is used in a sensible way like this I don't mind doing it however I am sure that's not alway the case...