Didn't New Vegas teach Beth anything? That game nailed followers...

- all followers should be essential(IMMORTAL) unless certain conditions are met, Only killable if a quest requires it or if not following you.
- ebony blade is fueled by killing followers. Fine, kill them after you dismiss them.

They are unkillable. When they get to 0 health, they fall to one knee. If they take damage after that, then they die.

However, I'd support on the easy setting they be completely unkillable. And on the hard setting, they not drop to a knee but die outright.

There's a mod for that:

http://www.skyrimnexus.com/downloads/file.php?id=472

I dunno, there are so many follower choices in this game, that if one dies, you can get another one.

I hardly see a point in having non-essential followers. Those that do not value their life will still want to have them around to avoid need of finding a new one. Those that want to have them alive would just end up reloading anyway if they die.