Yes it has the same problem as Tear in that it doesn't give any combat stats. But for Jayce and other AD casters who would build Tear, Idol has approximately the same mana regeneration but trades the stacking max mana increase for some CDR. If Forbidden Idol built into Essence Reaver, I could see myself getting it instead of Tear of the Goddess.
I suppose what we need is some sort of analogue to Vampiric Scepter that builds out of Longsword or some sort of Dorain's Reaver item that returns mana on hit. It would have to grant 80 AD when fully upgraded or no one would build it, but the intermediate parts would have to grant a mini version of the ER passive that is both significant enough to be worth building while not being overly powerful.
E.G. something like the following:
Spellblade = Longsword + Sapphire Crystal + 40g
+10 Attack Damage
+200 Mana
Unique Passive: Your basic attacks restore X% of the damage dealt as mana.
Also, what if Essence Reaver functioned similarly to Bloodthirster but converted excess healing to mana instead of to a shield?