Eye is a really hard shot, but doable from close range some of the time (but yeah wouldn't suggest going for the head at all. Even the leg is a better target as you could cause someone to bleed out quickly and if still the leg is a big target (could slow em down as well).
Still say chest (lung/heart etc) or throat is the best bet, either could be deadly and are fairly large targets. If it's just casual dudes possibly with guns etc but not expecting you (Ie no body armor etc), you could enter, throw the knives and leave/take off, causing massive damage behind you, get around a corner and a bit back after running and then hit whoever chases with a few more knives as well.
Still wouldn't say it's the best weapon and you need more practice then most other ranged weapons but yeah, totally silent (other then the people yelling you just threw knives into), Also VERY re-usable, so if there are just a few people at a time, you could possibly kill em, reclaim the knives and move on/repeat. Safer then using a melee weapon against 2-3 people per room, quieter then most ranged weapons and no worries about arrows etc breaking etc (so no ammo worries).
Back of the neck works just as well as the front btw, so if your silent/careful you could take out 1 or 2 people before they even know whats happening sometimes.
I don't believe that people are necessarily at a high risk of quickly bleeding out from a throwing knife, and unless you hit a vital area -- or the person gives up after any injury -- it doesn't really have a lot of stopping power.
As for the back of the neck, keep in mind that we're discussing defense >_>
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(\/)(\/)|-| There are precious few at ease / With moral ambiguities / So we act as though they don't exist.