I think this can be considered the thinking behind the starting changes from the man himself:
'McGahan believes that the Club needs to embrace their full squad, and keep all the players connected and hungry.
"We want to keep the squad connected and give blokes the opportunity to play," McGahan said. "It's a great chance to get guys involved in the competition early, and give them connection to the competition and to the group. Shota Horie, for example has only recently arrived from Japan and we need to see what he can deliver.
The coaches are still finding out about the players; we can do as much training as we like, but we really need to see them in game situations and it is a really good opportunity for us to continue our development and growth as a squad. You need a full squad to do well in this competition; it's not going to be up to fifteen or twenty guys to get us through all of our Super Rugby games.
We are only ever going to be as strong as the bottom end of the group, so we need to make sure everybody is reaching and going forward." '
From
http://melbournerebels.com.au/news/article/rabodirect-rebels-make-six-changes-to-starting-team