Oh I agree. It does fit in with the popular conception and depiction of armies. (And I would concede that it is historically accurate, although I really don't have any specific knowledge to support it.) It's just... not fun within the context of the game to have your own armies damaging your ally's lands as you march across the countryside to help him fight off the Evil Empire. It adds a negative component to marching to war. We don't want negatives. We want more reason for people to march to war, not more reasons to stay at home. So, yeah, it may be realistic. But so was dying of dysentery, and we don't put that in the game.
Advantages:
Discourages gangbangs
Discourages blob armies
Encourages attacking realms closer to your own (well, you're not marching through allied territory if they're your enemy, right?)
