RFC seems like a useless item most of the time
Thinking about most ADC builds, I find myself never building RFC for any reason. Generally speaking, when I am playing a warlord's champion I'm not building blade of ruined king. It's usually on champs like Jinx/Cait where I'm rushing IE/Runaan's instead of typical bork rush. Seems like bork has more synergy with fervor and higher mobility (Lucian, Ezreal) whereas warlord's pairs well with higher immediate crit chance. On the latter crit champions, assuming they have less mobility, Phantom Dancer always trumps rapid fire cannon.
Is there some other benefit to RFC I'm missing out on? When should it be bought, if ever?
level AS. Has needed that for its entire existence. Then it would be worth picking because it's raw stats are better. Right now, it has the worst AS of the 
items, that+typically less valuable unique passive than some of the others=it's not worth buying.