User blog comment:DamonDraven/Oh, dear Reaver.../@comment-24420226-20141213000302/@comment-35229418-20141213143310

It may be good for the mid-game on Cait, but ADC casters right now aren't in the best spot when it comes to rushing items. AS hypercarries (Vayne, Twitch) rush BotRK first, while the ones with good AS steroids and/or burst (Tristana, Jinx) are great with the IE rush. Sure, you can rush IE on casters too, but being not that reliant on autoattacks, they're going to be outclassed by the others (unless they themselves possess their own steroid, like Graves). So, AS ADCs rush BotRK, while the bursty/steroid-owning ADCs rush IE. What can casters rush that improves their skills? Nothing. I found that rushing Brutalizer+B.F. before boots was the best option on ADC casters, so I combined those items into what, according to Riot, was made for them.

Basically, AS ADCs and casters would rush their sustain item first, while the ones that rush Infinity Edge will go for Bloodthirster later, which has become a late game item anyway.