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why flat AD quints over armor pen quints? Since almost all of the AD carries are taking flat armor seals, they start with ~25-30 armor already, and they'll hit 31 armor by level 3 at the latest, and that means from level 3 on arpen quints are more effective than AD quints.
In addition to that, it makes it much more beneficial at that point to go TBC over LW, and that's even before you factor in the greater atk and aspd (for an AD carry who isn't using skills a lot).
LW (AD quints) reduction = armor - 0.6(armor-21)
TBC (arpen quints) reduction = 31+(15*max(1-number of hits), 45)).
Means at 3 hits (30 arpen from TBC) you pierce the equivalent of TBC at 121 armor, at 4 hits+ you pierce 159 armor. And that's before you take into account the aspd and ad buff.
The usual counter argument goes along the lines of: "tanks get 200+ armor". However, consider the incremental difference in damage (again, assuming only 3 hits with TBC, which vs a tank is likely untrue). At 200 armor, you deal damage with TBC as if they had 139. Damage = 1/(1.39+1) = 41.8% compared to LW, where you'd deal damage equal to as if they had 107 armor (~48%). The increase in damage from TBC over LW is (1.48/1.418) = 1.044, or a 4.4% increase in damage. at 300 armor the increase in damage increases to (1.37/1.29) = 1.062, or an increase in 6.2% damage.
Pretty sure 30% aspd and 15ad > 4.4% or even 6.2% increase in damage, and that's vs a target with 200 or 300 armor, meaning vs everyone other than the tank you're dealing more damage with TBC than with LW. Even if 2 or more people on their team go 300+ armor (very late game unless your team is stupid and all AD), you can sell TBC +200 gold = LW, for a net loss of only 1k gold (compared to if you had started with just LW, which would have put you at a disadvantage for most of the game).
All in all, I believe that arpen quints prove to be better in both the early and late game, due to the mechanics of TBC. If you refuse to use TBC, then why use arpen reds?