This idea has some merit IMHO. The devil is in the details, as usual.
/Edit: Actually, thinking about it a bit more, the current abstraction isn't necessary as bad as one may think. Armies do after all have contingency plans concerning everything from marching to the next town to invading Jupiter. Many tasks you set armies to do, particularly defensive tasks, would probably just be implementing already existing lower-than-you-level contingency plans. The major issue is really massive operations like Overlord, or Barbarossa.
A thought... how about the bonus scales between the issuing of the order and the execution date. Further more, the higher level you issue the order (Army Group vs. Army Corps) the better the bonus (scale) is. It would give incentive to issue higher level orders to army groups, with the option of micro-ing if desired. Sure go ahead and move every division if you want, if you think you can micro better than the AI without a 5% combat bonus
Would also simuluate Hitler's micro-ing screwing over the Wehrmacht and preventing them from fighting as effeciently