If I might just add something else to the discussion:
I think people's perfectionist tendencies might be leading them to the conclusion that NPC icons are "necessary".
You're all so used to the idea that you must never get spotted, and that you must achieve a perfect score, that you're not allowing for the fact that being spotted, occasionally, is a part of the game. Being led back to a an unrestricted area is a part of the game.
I aimed to create a true, purist experience, and that, naturally, implies a far more difficult experience than the one you're all used to.
Should it, not, then, be harder, and require greater patience, to not get spotted in a level? Shouldn't those who take the time to figure out their environment properly be more greatly rewarded? As I've said many times, I aimed to encourage patience, as well as speed.
I'm not going to get into a huge debate about this here. But that's my final contribution to the topic. Something to think about.