Take it straight from the movies: http://www.youtube.com/watch?v=9VTk2v5jGrc&t=7m0s
In a nutshell, giant NPC raiding parties. Would be nice to fight more things with guns besides immobile turrets and the occasional NPC who wants you dead because you picked the ugly trait.
On a side note: Npc’s on vehicles? Is it even possible? Imagine the mayhem