I personally never wanted killable NPCs when it came to them dying for stupid reasons like random town attacks. Those same people were always crying out that it should be like Morrowind where every NPC can randomly die, but in Morrowind, the only town attack that ever randomly happened was from a stray cliff racer that couldn't hurt a baby in the time it took the guards to take it down, and quest NPCs were mysteriously put in safe spots where no enemies roamed, and the NPCs never roamed into unsafe places unless they were escorted into them. So yes, the *player* could kill the NPCs if desired, but they didn't just randomly drop dead all over the place.
In Skyrim, I'm having to play my vanilla console game without Dawnguard or Dragonborn installed, and hold off on starting the main quest, until I am able to get all the main game quests done or else I'd end up having empty towns due to all the random attacks from higher and higher level enemies. It's not like the towns are all that full to begin with, and random attacks wiping out towns (and all the quests and merchants therein) are far less fun for me than having thriving towns full of unique people and things to do.
While it might not be a "glitch" in the strictest sense of the word, I do consider it to be gamebreaking in many ways. I've had no qualms whatsoever in using the console in my PC games to resurrect NPCs, make them essential, and/or use mods to make them behave rationally, in the sense of running away from combat rather than trying to beat on powerful enemies with their puny fists and iron daggers while wearing nothing more than a fancy dress.
I've heard the whining start up on the Fallout 4 forums about essential NPCs in that game, but I'm grateful for them, especially considering there are even less quests and unique NPCs than in Skyrim. I can actually *use* companions in that game for once.