It could be that he's waiting for the core of the team to actually reach their peak potential before resting them.
There would be nothing to gain by dropping a pivot who is still learning the game plan and where his support players are (hopefully).
or a forward who's not running the required lines because of a lack of understanding.
Once he's happy a player is upto speed with how (and where) he is expected to play, then you have something to compare others too.
Then you can decide who is the best player available for a role, or be confident that any new player will understand what's expected.
Judging by his quotes in the press I don't think we are playing quite how Sheens would like at the moment.