User blog comment:Sjjtitan2019/Itemization and Build Paths/@comment-6000413-20140110154615/@comment-35019468-20140111185657

AD build itemization is extremely narrow. All ADCs build IE / Zeal upgrade / Berserker's Greaves / LW / BT or BoRK / Defensive item, with nearly no variation. For non-ADCs, there's Hydra, Maw, Black Cleaver, Frozen Mallet, Youmuu's (niche item), Manamune (same), Wit's End (not commonly used), Zephyr (same), Atma's (same). Most of the time, you can't switch out the staple items for a more defensive item; ever heard of putting Maw on an ADC?

With AP itemization, you can easily integrate some defensive stats (RoA, Abyssal, Zhonya's, Rylai's, Liandry's), mana regen (Athene's, Archangel's), or cooldown reduction (Morellonomicon, DFG) into your build without losing out on too much damage.