No not unnecessary, because the warden title is associated with military command over additional armys, some kind of additional levys from the other lords in the warden region (north, west, ...) This could be included in this mod by getting a bonus for levys (it would be nice if this could be restricted to the wardens regions too).
And such military bonus fits very well in this type of game/mod
Hmm... you raise a good point. Let's see how the devs solve it, I admit I have no good ideas on the matter.
Is there a way to configure levied armies to replenish their numbers overtime, while called up, in a manner similar to mercenaries?
Might be worth looking at if you've got large levied armies from the North Marching south, they are going to be looking at a fairly large disadvantage when forces in the westerlands or crownlands, can dispearse and re-up fairly quickly. If that could be changed it would also allow for longer campaigns rather than spending what would presummably be several months gathering an army (if you are the north).
And why shouldn't it be like that? The defenders should be able to replenish troops from their own lands. It should be harder for the attackers to bring men to the fight.