I'm guessing the Epic Marilith Chain is so valuable because it's a critical component in a maximum DPS build.

Let's compare the benefits to the values being discussed here.

It gives +4 Seeker over a Bloodstone (or +2 over an Epic Bloodstone) and frees up the trinket slot.

That's a grand total of 4-16 (or 2-8) extra points of damage on critcal hits from the armor alone.

I guess with a Litany of the Dead, Greater Bold Trinket or Epic Gem of Many Facets you can squeeze out another few points of damage at most.

That is a very expensive minor DPS increase.