I think you're not seeing that essence reaver actually does have an unique identity.
First of all prior to essence reaver any ad champion that needed mana had to build tear or frozen heart or something. Which is very odd considering the amount of offensive ap mana items. Basically if stacking a tear wasn't efficient on a champion or was just tedious you didn't have any way to fix mana issues. So the item is inherently rather necessary.
Also, I think while essence reaver seems to not be popular, that doesn't make it bad. The league community closely follow streamers and the pro scene and if anyone famous uses it you won't hear these complaints anymore.
Most importantly, what I'd like to say is that essence reaver is not just an option it's a good option if you know what you want. Once you built up a shield bloodthirster definitely gives you the edge in a duel, however mana issues are a huge limiting factor that take their place whether you want to admit it or not. Essence reaver on lucian for example, (Not gonna lie, I only really play lucian adc) essence reaver is:
- Cheaper
- Gives you enough mana to use q and other spells much more often with no consequence
- Give cdr, an underrated stat on caster adcs
With this item Lucian gains the ability to spam his q much more often offering good poke and wave clear which can quickly overwhelm bt shield advantage. (Which is the only advantage in a fight btw they both give 80 dmg.) Also for each spellcast of course lucian gets a double shot and the dmg for the spell so the cdr is greatly appreciated on lucian.
With essence reaver and a good head on your shoulders you can often find yourself putting out much more pressure than your rival with a B.T. because they eventually feel pressured to use less spells and want to return to base because of mana issues (looking at you graves) while lucian can keep applying the same amount of pressure.
The last main reason I buy this item is that it allows you to use spells to farm camps and creeps faster without having to base. At all. Ever. This is make it an extremely good gateway item for farming up secondary items and participating in more teamfights and drags.
I realize that essence reaver gets torn apart by a lot of famous players and has a very bad reputation so I'm not going to sit here and argue with you about why essense reaver is best because it's not. It's different. 10% lifesteal and shield (B.T. advantage) vs 10% cdr and lots of mana (Essence advantage) But no one on this post seemed to really be able to find a use for this item and as someone that uses it a lot I thought I'd share my perspective on it. Even though I only build it on Lucian. Although I will admit that the item doesn't help out any champion quite as much as Lucian and Ezreal in my opinion simply because these champions specifically are ad caster that deal a lot of on hit dmg to replenish their mana with and there is a direct correlation between how much mana that have and how low their cooldowns are and the dmg they dish out in a teamfight. You can try and be conservative with Lucian and ezreal's mana pools and do just fine. (Most people do.) But you can't deny the increase in dmg output from being able to spam so hard.
I'd also like to add that if the mana portion got buffed, it would become a lot more viable on other champions such as Graves, Corki, Yorick, and Jayce. Which would give it a larger strategic niche in which you can use it. Although I don't think that's necessary because a lot of items are unpopular or highly situational and evening out the use of these items isn't really a priority to make the game better and often just shifts the the balance of power in unpredictable ways. The only reason to buff or change items in the game are really to buff or nerf champions in the game that use it or to make other item paths more viable. And Essence Reaver seems to be a perfectly viable build bath even though it's not popular.
I also may as well include my build path because it doesn't make sense for me to recommend a single item without the build path I think works best for it. And who knows, some brave souls might actually want to try it
- Essence Reaver
- Cdr boots (Eventually with Furor enchantment. You run very fast after an auto with this and tri and it's very easy to keep up with people.)
- Tri-Force
4.Last whisper
- /6. Then any combination of:
- Bloodthirster (if I need to get a bit tankier but still want to focus on dmg)
- Mercurial Scimitar
- Youmuu's Ghostblade (I rarely get this buy it's particularly good vs master yi as you can pop this and run at the same speed as him so he can't get autos off while you channel your ult. A full channel of Lucian ult especially with yomus popped is enough to kill a squishy master yi even if he's fed enough to 2 or 3 shot you.)
- Banshee's Veil
- Infinity Edge (For the highest burst dmg)
- Guardian Angel
- or Blade of the Ruined King (For tank busting. Which you'll have to do often in this cinderhulk meta.)
I realize this build is very off meta and it's basically an ad ezreal build but it works and it works well. The tri-force does a very nice job of giving you crucial defense stats you would gotten from B.T. and make him scrappier while the zeal is crucial for letting you use your double shot procs faster and occasionally crit which is very nice. And the sheen is extremely nice given the amount of spellcasts you make in the build and the fact that you will be auto attacking after each of these casts. The CDR boots are just to help you further abuse the sheen and near infinite mana and allow you to do to dash, use you q and w double shot procs and then have your dash back up. Allowing you to relentlessly combo enemies with spells with no clear weak times when your spells are down.
If anyone has any question or comments of course I'll be happy to answer them but like I said I won't argue with you as to which item is better. They're different.
Wannabe Essence reaver:
(875g) + 475g = 1,350g +25 attack damage +10% cooldown reduction Passive (unique): Killing a unit restores 6 mana.
Essence reaver:
(1,337g) +
+ 
+ 863g = 3000g
and arguably 
+
I wish it were a hammer
+ 
And the stats to:
)
B: have other ways of sustaining themselves built into their kits (I.E. 
Heavy Breathing
, for example, is a caster/autoattacker hybrid item that does just that). The idea of sustain on Essence Reaver also makes sense, because you're already purchasing it for mana sustain.
