Personally, when I want to lead an army, I load up Medieval: Total War 2. When I want to play an RPG, I load up a TES game. A crossover would only hurt the RPG elements.
I agree with you almost completely. However, some other RPGs have had great additions to them by including some form of RTS element.
If you could direct a battle- "archers go here, etc., etc., etc.", you could easily make the battle feel more enjoyable.
Granted, I'm biased- it always seems like most of my mods for Morrowind or Oblivion involve some form of world-changing big battles. Including my latest unannounced project that will probably never see the light of the forums (being that I'm out of time to work on it properly and starting to run low on ideas).