It's not about getting more damage in raw AD, or having lifesteal or AoE. Many of the champions that use Black Cleaver have some other form of sustain or built-in AoE. You're looking at the item in a vacuum.
It's the fact that in addition to maximizing CDR, getting additional tankiness, and taking 30% of your enemy's armor off the top, you're also getting a chunk of AD and a chunk of flat penetration. You're getting everything your champion needs to deal as much damage as possible, without really sacrificing survivability or a critical statistic.
For example, when a caster goes for the most possible AP, they sacrifice utility, survivability and CDR.
When an ADC goes for the most possible DPS, they sacrifice utility, survivability and CDR.
When an AD Caster goes for the most possible damage... they sacrifice... well, practically nothing. Stack Black Cleavers, get tankier, max CDR, ignore enemy defenses (except HP which is far more expensive) for the entire team, and get more raw AD. Oh wait, you sacrifice mana - a nearly limitless resource in Dominion.
That's the issue. I don't think this would be much of an issue if you remove the HP from the item... as I've said before, I don't believe it should be there in the first place.
Even if the item isn't actually overpowered, it limits build diversity.