Originally Posted by Epictues
and 1 support and 1 damage dealer dont do more damage than 2 damage dealers.
While that may be correct at end-game with full builds, you're not considering power scaling throughout the game.
Carries need items to get that crazy damage in the mid-late game, there are only 4 revenue streams for gold income. 3 lanes and the jungle.
Many carries aren't efficient at clearing the jungle. You can't have carries in every lane, you need some tanky people to soak damage and initiate, so really you only have 2 lanes for carries.
The current strategy is to put them in mid and bot(with support).
Mid because it's a short distance between towers and a short walk back to lane if you get killed.
Bot has 2 because it's closer to dragon so you get better early game dragon control with 2 champions near by than you would with only 1 champion there.
Because top is a solo lane, you need a champion that can survive ganks well, so you want someone tanky. Your bruiser or tank usually goes up there.
So that leaves a carry and a support to go bottom.
So, Mid and Bot have the carries.
Carries need 2 things to become stronger: levels and items/gold.
AD carries scale pretty exclusively with items as they just want to auto attack all day for max damage output, their abilities are less important than an AP carry.
So the Midlane will level faster due to it being a solo lane. AP carry goes mid
AD carry goes bot since they care less about the levels.
The support doesn't really need gold as much since they're used for heals, CC, and auras.
So to address the quoted text, if you dump 2 of your ADs in bot lane, each of them won't be as fed as an AD+Support. The AD who has a support will have that Infinity Edge way faster than either of the other 2 ADs. And having that big ticket item early (with a support who can CC and heal) will beat out 2 under-farmed ADs any day of the week.
Consequently, if the duo-AD composition decides to let 1 AD farm and the other to hold off, then the one holding off is going to be virtually useless because an under-fed AD is pretty much the definition of useless in this game.