I would love this. Unlike Warband your companions cannot increase their intelligence by bashing in someone's head and also battle sizes are too large to simply put them at the end of your troop list. Only choice you really got is put them into their own combat group and manually tell them to piss off at the start of each battle.
I don't know if Bannerlord pulls the same thing as Warband, where you don't get a companion's bonus, if they heavily injured, but eventually when perma death for companions is added to the game, having them be taken out in every single fight, would be super annoying.
So yeah, I would like to have a sort of combat group that just straight up doesn't spawn in battles.
ALTERNATIVELY, and this is a pipe dream but, you could include baggage trains in battles, then you'd have
a) a combat group that defends the baggage train and non-combatants
b) have a secondary tactical target to deal a massive blow to the enemy's morale
But that would be a lot of effort to implement and potentially impossible to balance, particularly with the AI.