As there seems to be a lot of people behind the scenes during SC's shoots (at least two cameramen plus the lady "straightness" consultant, plus the lighting crew - they seem to use a lot of lighting equipment - plus gaffers etc) it would make more sense to book them for two days ahead of time rather than send them home after one day. "Sorry guys, we're done".
As the BTS sounds heavily scripted some times, it is a lot of stuff to shoot in only one day. Except when there is no BTS (a distressing occurence lately).
Sorry I'm pushing for my theory, but in my mind, it means that the models get along at least a couple of days and that makes them more sympathetic, not just sex machines who can withstand "cameraman douche".