First and foremost: This is in progress, pretty much more like a scaffold about how I think Ashe is probably built best in Season 6.

So, why Essence Reaver? I think, with the changes to Ashe's ability to Crit (She doesnt do additional damage with crits, rather only doin improved slow), I really think that the Infinityy Edge is inferior to other Damage Items on her, cause the passive doesnt do anything.

The question is: when do you wanna crit? The most important crits are those when you wanna kite. Since you'll have either your Q running or your W available, chances are you will crit and thereby apply that improved slow pretty much permanenty even with lower crit chance values of like 50%.

Given all that: Ashe was always an ADC that highly profited from AD, even over Attack Speed, because of her W (some may remember the Blue Ashe build of Genja back at the end of season 3). Essence REaver really compliments on that: Massive Cooldown Reduction and mana sustain, so you can just spam your W in team fights, doin tons of AoE Damage. Also, the reduced CD on your Ult is wonderful.