As we all know movement of large armies is slow unless you use a lot 
of nobles. But if instead of executing/ransoming or otherwise dealing 
with any prisoners you just dragged them around then they become a 
great asset for a conqueror with a large army.
Lets take Cernennos' latest report as an example.
30 archers, 104 crosbowmen, 41 riding horses, 93 Pike, 30 soldiers, 
66 swordsmen + assorted others  total men 387
counting his prisoners he has 16 nobles in his stack. This means his 
movement penalty is two days per move. Not counting the prisoners he 
has 5 nobles. His penalty is 7 days per move. That is a bit 
different.
I think this could give rise to some bad habits and is not essential 
to the game balance. I think we should nip this one in the bud