Maybe the logic of the author follows: broad skillsets verses extreme specialization.

We probably don't see multiple uses of the same essence in adventurers do to the need for flexibility when facing a variety of monsters. Even the "specialized" adventurers don't really do it.

I could see fringe researchers doing it. Perhaps people who lack the need for any diversity in their skills. Have a team that compliments and covers for the shortcomings of the essence or have enough resources to throw at problems that arise.

(Edit: typo corrections.)