Doran's Blade's and Ring's passives are stackable - Why not the new Doran's Shield's too?
When you are behind or in need of early game power to keep up with your enemy, stacking several Doran's Blades and Doran's Rings can be very efficient.
A midlaner with several Doran's Rings will be able to spam a lot of abilities in lane to either outpush or outpoke his enemy, I even remember the 2-3 Doran's Ring combination being called "The Beggar's Rod of Ages" because it would be exactly that in the early game - a combination of health, mana and AP.
Same with Doran's Blades. Stacking several of these alongside a bunch of long swords will give you a shitton of raw AD and HP in the early game to outtrade your opponent - or go even if he is a lane bully or ahead of you. Not only that, the life steal will also allow you to regenerate the HP that you just lost by trading the enemy by simply farming minions, to get an edge on your enemy laner.
But stacking these items without you making proper use of them in the early game or the enemy simply denying you the ability to do so will make them less and less efficient in the mid-to lategame, because they will delay your full item power spikes and crowd your inventory.
And while I really like the new reworked Doran's Ring on the PBE ( http://www.surrenderat20.net/2017/02/223-pbe-update.html ), which will now finally fulfill its role as an early game item for tanks or just champions who don't want to or can't trade the enemy to just survive the laning phase, I think it's somewhat odd that it doesn't stack aswell. Let's say I'm a Darius into Quinn, getting outranged constantly and in desperate need to survive the laning phase and preserve the early game power that Darius is known for, being able to only buy one Doran's Shield and having to buy Doran's Blades afterwards is just kind of meh. Darius into Quinn is just one of these examples - there are many more.
At the same time, it would also be a great way to nerf Quinn, Jayce and many others ( if the lethality changes didn't do that enough already - we will have to wait for reliable data to develop later this patch ) without having to directly nerf them and risking that they get Sejuani'd.
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