I think a big reason for the essential tag is to keep them from being killed by random beasts, dragons,etc. Most characters spend some if not all of their time outside. It would be a bummer to have a quest essential NPC randomly killed.
That wouldn't bother me in the least, since it's supposed to be a dangerous place to live. I would
far rather lose quests because a giver got killed after wandering too close to a bear cave than be stuck with people being essential who have no business possessing that status. In fact, I made a 'No Essentials' mod for just that reason, one benefit of which is that it will actually possible for my characters to purge a given faction if they see fit (i.e. if their RP justifies it). Most probably won't, as I rarely play that way, but at least the option will be available.