It's hard to fit Roberts in if we're only picking 2 back reserves. He's effectively a 12 and nothing else.
While you could have Perese covering the back 3 from 13 (by being able to move to the wing) I also don't think you want your best back being pushed further out in the event of an injury (to a winger, fullback or 10).
I'm sure he's being a great mentor though