I don't even get the point of having essential characters. Morrowind didn't. It worked out fine. I'd rather have the ability to kill someone my character would kill, than have Bethesda make sure that I don't kill them so I can do a future quest that I currently don't know about, and, if I did know about, likely wouldn't want that character to do either. Tell me...why can't you kill her, but can kill the entire DB? Just..stop essential NPCs. If you screw up, and kill someone you shouldn't have, have a "the thread of fate has been severed" warning, so you know to re-load. It's stupid, childish rail-roading, and treats the playerbase as children who can't live with the consequences of their actions.
I'm tempted to copy+paste my standard multi-page rant on this subject, but instead I'll just say that I wholeheartedly agree that essential people not attached to the main quest should not be marked as such. The only reason I make an exception for MQ people is the lack of an alternative if one of them does die prematurely; I would actually prefer just letting the main quest fail if you kill someone attached to it, but they won't do that; as such, if we choose to kill MQ-critical people, provide some kind of alternative method to allow the MQ to proceed.
It's especially insulting when the essential person is also an obnoxious git, since they neither shut up willingly nor can be forced to; since you can't avoid some of these people if you actually want to do business in their town(s), you end up either cutting yourself off from about half the merchants or having to deal with these losers while trying to sell your loot. One of the first mods I will make when the CK is released is one that changes the essential tag to the one used for companions; while I don't mind quests failing, I
do mind when it's because the idiot AI wandered off and got eaten by the local wildlife, and this will make sure that quests only break if/when I kill a person related thereto.