Carnage Matey,
What about the return flight ?
Yes they may also be booked as light, but we're talking about a flight 24hrs later most of the time. So many times, another carrier flying the same route home has gone tech, and we've filled up with their pax.
The only times I've left short of a normal complement of crew, was when BA were 'capping' flights.
F would be only half (I think) booked, and 2 crew. Sometimes F would be closed.