Pilots who take flight plan fuel if it is not enough to get somewhere reliable are setting themselves up IMO. I have no idea about this incident but at least once in a career an airport is going to close for a random reason ( bomb scare, lights go u/s , plane on the runway etc) and then you'l need to go somewhere else reliable. You need fuel from the miss to a runway where weather isn't a problem, simple as that really.
Ps that isn't a comment/ judgement on the Virgin flight, just on the topic of flight planning.