I can see what you're saying, but I don't think it's necessarily a good simulation of reality. In reality the plans may very well include things down to that level of detail, however they will also be constantly adjusted on the fly, meaning that in game putting plans that detailed would almost immediately result in the need for departures of SOME kind that in the real world would have little to no impact on the overall plan.
Additionally taking a single division and sending it 1-2 provinces over but still on the same axis of advance would put it drastically outside the planned supply lines or air cover areas, having it return to the regular advance afterwards would almost totally eliminate what disadvantages occurred.
If you have 100 Divisions under the command of a single Field Marshal and draw a single offensive Front line for those Divisions the game will consider it to be a Battle Plan and you will start to gradually accumulate a bonus however I would argue that its not really a Plan and that any bonus should be limited to reflect what is clearly a lack of any real Planning.
Lets assume that the above can only allow for a maximum bonus of 10% however if you also had under the command of that Field Marshal 3 Army Groups and each Army Group was assigned its own Front line the maximum bonus should rise to 20% because there is now more Planning and so on down the command structure which each level of command increasing the maximum allowed bonus.
To gain the maximum Planning bonus you would......
1\ Issue instructions for the AI at a Corps level, effectively your micromanaging your Battle Plan.
2\ Have as many levels of command as possible, a PZ Corps directly subordinate to an Army Group would have less of a Bonus then if it was subordinate to a PZ Army\Army Group.
Of course there would be no reason why the player could not issue instructions for the AI at an army level or at an Army Group level, the bonus would be less but perhaps you would gain more freedom to adjust the Plan when its being implemented without losing the smaller bonus.
Of course your right, when faced with the reality on the ground a Plan may need to change but you do not create a Plan with the intention of changing it, the whole point of a good Plan is for it to work.
At present the game play involved when creating a Plan is not great and as the OP pointed out you can take manual control at any time without losing the bonus which effectively means that the Battle Plan system is really only a movement system, a means to tell the AI where you roughly want your Divisions to go, there is no real military Plan.
I recognise that this discussion is academic since we do not have a command structure worth the name but perhaps further down the road Paradox will see ways to improve the system.