My guess regarding Mogg on the bench instead of Horne is to minimize the reshuffle in the event of someone needing to be replaced.
Whilst the biggest risk is that one of our centres (particularly AAC (Adam Ashley-Cooper)) goes down, I think the choice of Mogg over Horne is to avoid our centres having to move if Mogg comes on (at any place other than centre).
If a winger goes off I expect Mogg will just play on the wing. If fullback goes down, Mogg will play fullback and if JOC (James O'Connor) goes down, Beale moves to 10 and Mogg plays fullback.
If Horne was the player on the bench then the only positions he can play are 12 or 13 and any injury will require us to shift our centres.
I think the underlying thinking is that the only reason we want to shift AAC (Adam Ashley-Cooper) out of 13 is if AAC (Adam Ashley-Cooper) gets injured.