So how does each deal with the killing of an NPC who would give a plot-important quest? Or is the main quest given right from the start?
You can find ways to deal with it if you realy want to. You can make quests failsafe by leaving a note on the body, or leaving quest reward reachable after death of the NPC. Or your npc may not die in a plausible way, like teleporting to escape, a cutscene that saves him from players grip.
The main question is what do you gain from killable NPC's and how commited are you to the concept. Because it will haunt you all the way through the development as you implement guidelines to support this idea. It may also cause problems after release when you forget something and the players get stuck because of their ill decision.
I am personally in favor of attackable NPC's. However it is not without negatives. I have found the idea of killing NPC's only meaningful when the game permits player to impact the world. Otherwise disadventages quickly pile up against advantages, on a developer's point of view of course.
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