Think about it this way... it is fairly cheap for FR to over-crew their aircrafts considering 72% of pilots are "pay as you go" contractors.
FR quite well knows that 99,9% of these contractors feel absolutely no loyalty to to MOL the almighty... So yep to the best of their ability FR try to have a few extra F/O's available... this caters for the increased summer production and for when now and again a driver goes "awol" without giving notice.
The extra number of F/O's also increase the captain candidate pool, which will help FR drive down the T&C even more.... all this with the only real and modest cost being the sim sessions for these extra pilots...