Hi Botor,
I'm definitely not insisting on horde formation in all cases, or denying that there are any situations where the bus is superior. I'm just saying that, in my experience, these situations are rare. The one you bring up, where a horde is combo charged, I think is a good example of the bus being superior. And it goes without mention you have to leave horde formation if you need to fit through gaps, or you have a big army deploying in a battle for the pass.
Charging monstrous infantry along with cavalry support as you mention is an interesting example. In this case I don't see the advantage of the bus so much, you'll break the monstrous infantry steadfast with a horde just as easily (and be much more likely to win combat), as they're seldom more than 3 ranks. Now in receiving a charge from monstrous infantry, being in a bus could well be better if the monsters are six wide.
Another case to consider is monstrous cavalry. 4 Mournfang are going to do just as much damage to a bus as a horde, we're going to be steadfast nearly as long, and in a horde we'll at least grind them down some.