Why are some Skyrim NPC's marked essential?

[wordsmith]

It does kill the immersion for me. But I understand why they did it.

I don't. Think of how immersive the game would be if your questi could be ended by a random dragon attack killing everyone involved!

It is a losing battle trying to get immersion in Skyrim, anyway. Whether it be the randomized guard dialogue (referring to me as a new member of the Companions despite me being the leader), the game not being able to keep up with your accomplishments (A woman threatened to call the Dark Brotherhood to take care of me if I got out of line. I was the leader of the DB. There was no option to burst her bubble), or just encountering glitches that remind you that you are playing a game.

Now THIS is something that bothered me. I did the Thieves and the Dark Brotherhood, I just got around to "restoring the Thieves Guild to it's full glory", and some punk Dark Elf joined. He mentioned something about being ex Morag Tong, and if I said anything to anyone, the Dark Brotherhood would come after him. I took exception to him saying "Keep your mouth shut, unless you want the Brotherhood in here", and as I was unable to say "***** please, I run that", I was forced to punch him repeatedly until he stopped talking. I didn't mean to kill him, just stop him talking. Permanently.

 

[Something Amyss]

It is a losing battle trying to get immersion in Skyrim, anyway.

Then you obviously aren't playing the child-killing mod.