I am not sure what other use Quintesence of Knowledge has, but I do know that it is a reagent used with the summoned Mage item 'Talisman of Return' the talisman is a 1 time gate to PoK and you have to have Quintesence of Knowledge on your person for it to work. Is a long ,20 second I think, cast time so it's best used for classes who can't gate and wish to travel to desolate places without porters. In a pinch, I have had the Pally I was duoing with start her gate while I kept pet healed long enough for the gate spell to get her out, then cast mine which has a much shorter cast.
It's a pity that they made the Talismans NO RENT it almost makes the spell worthless, compared to Gate Potions.