I don't understand the power of the Dark Knights

[Cilia]

Then remember that the "Darkness" it's referring to is "Spiritual Darkness" aka your character's inner bad guy.

Not totally accurate. From a psychological perspective, if what / who you're referring to is the "Fray" portion of the Warrior of Light's psyche, s/he is far from a "bad guy." "Fray," like the characters' shadows from the Persona franchise, represents everything the Warrior of Light is but denies: that s/he often fights just for a sporting battle and to test his/her abilities, that s/he hates being roped into helping idiots who could have helped themselves, that s/he hates being pestered into helping others do their jobs, and most importantly that s/he is in great emotional pain from everything during 2.x.

Remember, even in Persona 4 the characters' shadows aren't evil per sé - just a representation of their repressed emotions and desires. The only character whose shadow is truly evil is the main villain, and as an attempted rapist and murderer, that's hardly surprising. "Fray," or your spiritual darkness, is just your emotions and desires running wild, with no mask of society to keep him/her in check. The worst s/he does is beat up some soldiers at Whitebrim Front to stop you from being executed (since, y'know, Ishgardian justice is "die as an innocent or die because you're guilty"). You would sacrifice everything to protect the world; "Fray" would sacrifice the world to protect you and make you happy.

The trick to understanding the power of darkness is the ability to recognize and accept that "Fray," or your spiritual darkness, is not your evil impulses. It's not wrong, or bad, or evil. It just is - it is you, all of you. That strength of heart required to face oneself is why so many probably fail and die in the process of becoming a Dark Knight, and what happened to the Warrior of Light was probably a big mishap that went on far longer than it should have.

... not so simple, I know. Still, it's better than having "Fray" go on about how s/he wanted to have sex with Minfilia and/or Thancred, and now s/he can't because they're MIA.

We will just have to wait until 4.0 to see how the Dark Knight job quests continue

Maybe. If you talk to Sidurgu again and ask how he and Rielle are doing, the Warrior of Light will suggest that he train Rielle in the dark arts - a suggestion he is flustered by, but is open to if it's what she wants. So it sounds like we might be training Rielle...

Could be a red herring, but still.