I wonder if perhaps Unicron's armour is all Megatron has, or whether he may not have absorbed portions of Unicron's spark, in a similar way to Starscream stealing portions of Primus's power.
Certainly the thought makes the battles between Starscream, Megatron and Prime far more symbolic if this is true. It would also help explain the reformatting into the Galvatron of this universe, echoing the G1 story.
Or am I reading too much into that? -TTK