Bennett is not afraid to leave players unused on the bench - he did it all the time when he first started at the Dragons. It's a huge benefit to have second rowers play 80 minutes, and the more the better. Opens up more interchanges for props, and allows more leeway for injuries.
They were normally rookies though. not first graders with 100+ games experience. It wouldnt suprise me though, to see the 4th person on the bench to only see 10-20min a game though.