TdF is so much bigger than all the other races from a sponsor's perspective I can't see Sky being happy with a situation where Froome rides the Giro and thus compromises his ability to with the Tour.
Froome riding the Vuelta having ridden the Tour is a no-risk option for them
I was wondering this too. From his personal point of view I would think he'd want to get a Giro win in the next couple of years before his powers start to wane, but as you point out this may not be in Sky's interest. Perhaps he'll have to change teams to have a crack at the Giro