Why not build Essence Reaver on Ashe?

the mana regen means you can leave her Q on permanently during teamfights

You can do that without Essence Reaver.

You won't be able to finish Essence Reaver during the laning phase. When the laning phase is over your manapool will be large enough due to your natural stat increase.

Bloodthirster give you better stats for the gold and the build path is identical.

Teamfights shouldn't last all that long, and unless you unloaded all of your mana on your ult and w, BT would help to keep you alive better and you don't have really super important skills for the 10 CDR. If you're really fed, then I guess you can go ahead, but BT is a better choice.

I almost always build ER on my ADC's because of 2 reasons: lifesteal and sustainable ability-use. The damage on ER is decent, and frankly, an ADC needs lifesteal. I always kit my ADC up to deal lots of damage and extremely high lifesteal - always over 33%. BOTRK, ER, BT, plus the little bit you can get via Runes/Masteries. Then, Crit items, 1 or 2 of which should also provide movement speed buffs. So IE as the main crit-damage item (always) and then the other 2 spots are situational but 1 MUST have a movement speed buff.

I typically look to deal damage and to risk my squishy champ on my ability to position properly and do tons of damage while getting massive health back per hit. The latest version of Ashe is even more of a beast now, so I think that Black Cleaver is a must on her as a 6th item. Look to get the dragon speed buff to supplement the loss of another movement item taken over by BC choice. BC isn't the last item, I'm just saying that it is a critical item to take on her to make her freaking OP. Trust me, a combination of BOTRK with BC and IE is so fearsome on Ashe now that almost any tank will only Face-check you once after you have that combination - they'll learn a hard lesson if you're good with Ashe (good = positional intelligence as a minimum requirement)

Essence Reaver isn't just worth it, goldwise.

It has a very high cost, similar to the Bloodthirster and Botrk, but it doens't give % damage nor a shield.

And it's actually not that reliable , you do run out of mana while kiting with the Essence reaver. Not to mention that by that point in the game, if you don't have damage then your teamfights will be really screwed (hence why a Stattik or Infinity Edge would give you the well, edge)

If you want a mana item that is good on Ashe, while still scaling well troughout the game go for an early Tear of the Goddess (720 gold), which then can be built into a Muranamana which gives you damage, mana and mana regen.