Hey, it's not my theory. If you can go by the PB on an F-150, the tank size drops to 30.6 gallons, so you basically lose 6 gallons. Then there is the added weight of the battery effecting performance, and whether the boost helps much in off-road conditions. It clearly would be a plus for having power available when you aren't driving.
Personally, I hope that it's something Ford is looking at doing in the Raptor in the near future. Perhaps Ford wants to have sort of a 'pilot' year to see how customers like it. Or perhaps they think many customers won't care for it and thus don't want it in the currently single engine option Raptor. We'll see.