Why would this not be desirable? I am certain that border regions in countries/empires did not produce enough gold to man the militia defending them. That gold came from elsewhere, why should we not do the same here? If a wealthy duke wants to keep the fighting away from his/her city, why not pay for militia to be placed in the poor region which would block access?
I guess because a lot of wars would become infinite stalemates: if the front is narrow and you can clog it with militia, war would boil down to the one that micromanages gold flow the better to keep said militia in place.
As things are now, if you want to win a war you have no choice but to dump money into nobles (players!) and organize them the best you can to face the enemy.
While perhaps the first could be more realistic, the second is definitely more fun. Especially for those who are not allowed to play with gold an militia (knights)
