I would only get 2xPD if you have a full build and feel like selling boots. Otherwise, I'm always in a bind, every single game, because I need GA, LW, and IE as soon as lane phase ends and I always have to make choices.

There's no good reason to buy two PD's unless you're playing something goofy like AD sion.

Generally speaking if you want to buy a PD, you also want to (eventually, or already have) buy an IE. A second PD is gold wasted on crit you don't want / can't use. There's better values for AS, and better values for dealing out damage. MS alone isn't a good reason to buy a second PD.

This is all fine and well, but this is not taking the game into account.

The job of the ADC is to kill front line tanks, fighter, and bruisers. 200 Armor is nothing. If a fat cho with like 4K HP and 300 armor is running at you, 2 PD's and an IE is going to do nothing, even with crits. A LW would absolutely shred his armor though and let you easily kill.

Another thing to take into account is sometimes you are only able to auto attack once or twice before repositioning, LW makes you hit harder against front liners which are likely trying to get to you and kill you.

This is why I go IE - PD - LW - GA.. It's a much better build for the job of ADC, which again is to get rid of the front line, while AP nukes the back. Of course you have to think for yourself in every situation, generally you are sitting there attacking the closest target because if you chase into the fight, you die.

Madred BloodRazor + Vayne Silver Bolts = The best anti tankies combo ever
Since Madred cost like IE you can replace it
So Having 2PD +BT + Madred Would be a nice combo: Magic damage also heals you so you will be getting alot of healing per second

And what about Black Cleaver? Remember that LW only works for your attacks, while the Black Cleaver reduces its armor so is more useful in a teamfight since all your teamates will get benefit from armor reduction (BC on Twich ulti works pretty well) so i would add a BC to the list.

If the AD carry have skills that scale off on AD and deals physical damage (Graves, Ezreal, Varus, Draven), the PD and LW combo works better.

If the AD carry relies mostly on autoattacking and barely have any skills that scale on AD that deals physical damage (Ashe, Tristana, Kog'maw), the two PD is much better.

Though with the upcoming nerf on PD, I'll be more inclined to the PD+Statikk Shiv+Beserker Greaves with Alacrity or Furor combo with Tristana instead of two PD.