With the schedules and the some times suicidal behaviors of NPCs all around if the essential characters didn't have essential status than half the time they'd be dead long before I ever encounter them or need them for whatever reason.
And sometimes, yes, I need it to protect me from myself as well as from the NPC's. There was a play through in which I was hired to clear out the vampires of Bloodcrust Cavern, by Hasildor. I managed to get the other Vampire Hunters to be whipped out, and I moped up the surviving vampires (a truly evil character should never get their hands more dirty than needed, playing both sides against each other and seizing victory is an essential practice).
On my way out of the cavern I saw none other than Hasildor lurking around in the darkness. Naturally I assumed it was a vampire mage I somehow managed to miss and so I struck! Wham-bam-thank you ma'am and down he went like a sack of potatoes and then I received a message stating that Hasildor had lost consciousness. I was never more grateful for essential characters than I was in that moment.