I'll simplify it.
% armor pen vs flat.
% = last whisper and such.
Flat = lethality.
Just so we're clear, lethality is what armor pen used to be, just scaling numbers over tim. Still flat armor pen.
Let's say I can reduce 30 armor with armor pen. A 100 armor target still has 70. A 30 armor target has 0. Because of this, the less armor, the more effective it is.
% armor pen is different. Let's say i can reduce 33%.
30 armor = Im ignoring 10.
100 armor = Im ignoring 33.
% armor pen is more effective with higher number values.
Flat armor pen is stacked on assassins to burst because it's insanely cheap. They dont care to go crit and attack speed (which dont scale with their abilities to begin with, but, ignoring that) to scale into late game because they dont want late game to even happen. They want to close games out and kill people hard early.
It's important to note item/power spikes. IE is a power spike, Shiv+IE is a power spike, Duskblade is a power spike but so are Dirks and BF swords and such.
You get more swift powerspikes with lethality making it easier to snowball out of control.
On top of that, well, like I mentioned previously, you dont really abuse crit/AS with assassin abilities. They want their abilities to hurt, not so much their autos.