ok then lets say his autos do so much damage.. lets say 90 base damage + 15 from runes and then we get an even 100 damage because fuck math I said so.
How fast does he attack? Doesn't matter. I mean sure he will attack so many times and then he'll have enough to kill the target he wants. Great, that's nice but lets just look at the damage
it doesn't matter what his attack speed is, if he gets this he will attack 50% faster but any AS after this stacks additively percentagewise blahblahblah
this gives him onhit damage to stack on top of his true damage. With this alone he'll be able to shred through your defense if he attacks fast enough and thus his defense penetration now scales with attack speed. Congrats but he currently has no extra damage
25 attack damage, 40% attack speed, more onhit, healing. This one gives an actve
0 attack damage, 45% attack speed, critical strikes, damage reduction. This gives movespeed and less unit collision
With 30% crit yi will crit once every 3 autoattacks. This is not actually random he WILL crit once every three autos.. actualy its a bit less than that so once every 4 autos. Its actually once every 3.3333333 autos but whatever. Anyways once every 4 autos he will critically strike adding 100 damage to his attack but this will basically had 30% or his crit chance to his damage. Or in otherwords it can be read this way
On an attack speed champ, crit chance is effectively a damage multiplier that can max out at 100% based on how many attacks it takes to realistically get the crit
So lets look at these phantom dancer another way
45% faster attacking, 25% increased damage from autoattacks and critting abilities, reduced incoming damage so you can get more damage in, etcetc
Now, since its 25% more damage we can say that this is the equivalent of an average of 25 attack damage. Which almost makes this kindof impressive but then you also have to consider all the other stuff.
on master yi attack speed scales a bit more than normal blahblahblah but here's the thing though. Now that you have attack speed and crit if you get 
for some reason you get 10 * 1.4 = 14 effective damage from alone but you get 101.451.25 = 18.125 effective damage from .
as you go on the difference will get bigger. Ultimately even if you never get anymore crit will always give better scaling with master yi himself and his build but also considers how tanky the opponent actually is and stuff like that and gives a sustain source.
Both have their uses, its all probably just situational but once you have any % hp item you have something to mitigate some defense with on top of the true damage.
Crit chance doesn't give more true damage but it doesn't have to. It scales multiplicatively with both AS ad AD. If you get 100% crit chance on yi you're probably doing it wrong . In fact to guarantee a crit every 3 autos you only need 34% crit chance.
Its all situational.
Crit kills squishies
true damage penetrates armor without a need for penetration items
% hp damage penetrates hp items, it scales with AD and attack speed, you can also use onhit amplification like 
to make it work harder
This is the use the crit has. Its an amplifier, its like the 
of autoattacks.
alone would give him tremendous ability to cut down tanks, not to mention 
. defensive itemization is also more supportive for crit Yi to return with the likes of 
and the current iteration of 
do, especially because the former build follows up with 