To repeat what I said above:
(In a way, defence against the Others and their creatures is an example of Ice and Fire working in tandem.)
** - This begs the question of what happens if the seas along the coasts freeze over.
While the Others may not be able to pass through or over the Wall (or across the sea**, given that the Wall ends at the coasts), those that they control obviously can. And they can (only?) be (re-)killed by using a fire. The Wall cannot wield a torch. That requires a person.Well, we know that Wildings can climb over the wall, we know that dead Wildings can be made into Wights and we know that Wights can operate on the southern side of the Wall. So Westeros does need the NW.
(In a way, defence against the Others and their creatures is an example of Ice and Fire working in tandem.)
** - This begs the question of what happens if the seas along the coasts freeze over.