User blog comment:Rapacious/Rapacious's sexy guide to understanding ranged AD builds/@comment-131.174.224.196-20120201222441/@comment-3450702-20120202163317

Note that you also gain about 50-60% AS (about 3% per level) at level 18, but almost DOUBLE your AD (from standard 50-ish to over 100). His calculations are correct. But IE is still too wonderful to stall by PDs. The AD alone from IE is worth PD, and I haven't even included the CrC and CrD. Then again, PD is also awesomely cost efficient.

However, most AD champs have an AD scaling ability, which makes AD worth more. The only real reason to get PD over IE is if you got lots of AD from a steroid, some on hit effect and/or no AD scaling abilities. I can't think of a champ like that atm (AD teemo? :D)