I've been debating this in my head for a while, specifically for the Ashe rework. I don't think BC is better than LW for most ADCs; most don't have a way to apply the 6 stacks quickly and would probably rather have an instant 35% instead of having to attack a champion for a while just to get 30%. AD carries are generally focused on dealing as much damage as possible, so outside a few circumstances, like if you're a largely AD team and you're playing MF or someone who can apply a lot of stacks really quickly, I don't really see it being optimal, especially over LW. Furor boots also give better ms, especially since phage bonuses are halved for ranged champions, so that's probably the optimal choice if you need to kite better. CDR isn't really necessary on most ADCs since they're much more AA based than most champions; it's definitely nice on many champions, but given the option, most ADCs will go for straight damage on their autos than CDR. ADCs are pretty unique as a class as they pretty much will single-mindedly prioritize maximizing their DPS over anything else, and more often than not their autos will be dealing more damage than their spells in the mid-late game.
The reason I'm debating this on Ashe, however, is because her Q will stack the cleave passive incredibly quickly, so she isn't losing as much as other ADCs trying to stack up the cleave passive. She benefits from the other stats as well; the extra health for survivability, the tiny bit of MS since she's pretty immobile, the CDR since her cooldowns outside her max rank W are pretty long. Even then, though, in a strictly optimization sense, I don't know if that outweighs getting LW for the more immediate/less situational armor shred, and just getting Furor boots to help kite and having a little gold left over to put towards a lifesteal item/more damage. Situationally it'll probably be very good; on an AD heavy team building Runaan's + BC just to shred everything in sight sounds very powerful, but I don't know if it will be core/default.