To answer the question for Dorans shield at the start i like it because it gives you 120 health 10 armor and some health regen. I prefer it over Dorans blade or even a vamp scepter because dornas blade only gives you 10 damage and 3% life steal the damage really is not that significant since by level 2 you will have your first rank of wuju style. Also the life steal from Dorans Blade or vamp scepter really do not make to much of a difference early because you have no attack speed or attack damage and you are not really getting that much use out of them. Now for the 2 PD i like to get them because the movement speed is awesome for Yi because it gives him the ability to chase just about anyone down while also being able to out run anyone as well. Also this gives you a 60% crit chance with the 2 of them and with the armor pen and infinity edge you are able to crit for about 500 until you get your bloodthirster then you can crit for about 650 with it fully upgraded. Also the attack speed from the 2 PD gives you roughly 1.8 attack speed which with your crits and life steal you can just take enemies down in seconds and constantly be stealing life from your opponents keeping you alive longer.

I really don't understand; runes, masteries, Doran's shield and double-PD's. I'd say that armor and mana regen are better than dodge for seals and alacrity glyphs maybe a bit weird ones. Why Doran? Why 2xPD and no AD in the start, you'r gonna have weak start. Well, there were couple points from your build. I am waiting for answers. Couldn't neither downvote you cause of the voting bug. ;P

Nice guide ;)