Simply enough, the closed cities and closed houses was mostly an response to very little memory on PS3/360.
Notice how FO;NV strip and even surrounding areas is divided, its more obvious than IC in Oblivion. Remove one of this walls and PS3 will have an significant chance of crashing.
This is not an issue anymore. No issue having 50+ npc on screen.
Again this has an downside, some chance important NPC dies because of random attacks or because player pulls groups of enemies toward town and forget to close the gate.
Open cities has been an way more common request than being able to kill everyone since Oblivion. This is also true for more natural behavior of NPC, we got that in Skyrim where people work outside.
So choose one. Loading doors before quest npc who never leaves the room, or protected NPC. Note they got pages up and down about complains abut vampires wiping out towns.