In single player I can see the use and against AI I see it has a helpful feature. As I said above I have a issue with bonusing AI to make it viable is something I'm strongly against. Its not creating good AI its creating boofazoo AI. But in MP....player versus player. Controlling troops will always beat not controlling troops...AI pathing among many things is inferior to player hands. The best thing the AI has is efficiency and all that really is being more attentive players and really just knowing far more in strings then the player because its not apart of the program. My point is making plans I feel won't work and will fail if one side is controlling units directly.
You are basicly concerned about a balancing issue. How big will the Battle plan bonus be. It will need to be high enough that loosing it by manually intervening needs to be carefully considered but not so low, that doing everything manually is always the better option, therefore rendering itself useless. I can see this being a main point of contention for the whole lifespan of HoI4.
If a plan fails, there will be much debate if it is the fault of the AI, or a poorly devised plan by the player. Don't forget, your battle plan is basicly telling your troops what you would do roughly manually yourself anyway and if you have to deviate from it quite substantially it might not have been such a good plan in the first place. You are not giving everything to the AI, the AI will try to do what you told it to do and will act in these limits.
If they pull the battle plans off well, even in multiplayer you will want to use a battle plan against your opponents for the bonus, and both players will have to intervene in their plans at certain points. At which point it is best will possibly be a main theme of the game.
It is a difficult task they have set themselves, so i get the scepticism but i think it is possible.
Regards