I just think that in a game where they take pride of player freedom so much, having essential characters is just too restrictive. In Morrowind you could kill everyone, if they were relevant to the main quest a popup window would appear saying something like " With the death of this character the prophecy can't be completed. Load a save game before this character died to restore the threads of the prophecy." or something like that while the player could still play the game after that character was dead. And I think that even by killing those characters the player could still beat the main quest if he killed Vivec or something like that.
Morrowind might have been more restrictive (the way RPGs are) about what a particular main character can access or do in one playthrough but without a doubt gave more freedom about characters deaths than Skyrim ever did. There was none of this cheap and useless "Essential can't be killed" lazy gimmick.
EDIT: Found an image of the popup message: