“That’s the problem with you nearly immortal types,” I said. “You couldn’t spot a pop culture reference if it skittered up and implanted an embryo down your esophagus.”
There's a recurring element in Dresden Files in which Jim Butcher will use pop culture references as a means of demonstrating whether a character should be considered good, likeable, or trustworthy.
Harry Dresden makes a lot of pop culture references. References are fun, and they help the reader feel more related to Harry. Likewise, these references are how Harry can relate to his friends. It's one of the primary ways he enjoys joking around with them.
However, the antagonists of the series, the vast majority of whom are supernaturally long-lived beings, have not bothered keeping up with popular works and media of the last century. They haven't bothered maintaining connection with the everyday human experience, the art and pastimes most people have in commonality and can bond over.
Madeline Raith has never heard of Buck Rogers. Nicodemus doesn't know who Peter Parker is. Drakul doesn't even know how baseball works.
This lack of interest in pop culture is equated to a lack of interest, connection, or care, for humanity in general.
References can be a signal for the reader that a character is someone to be trusted or empathized with before the story even confirms it.
Thomas wears a Buffy the Vampire Slayer shirt a whole book before he's revealed to be Harry's family in Blood Rites.
In Skin Game, Harry and Goodman Grey have a good-natured back and forth about the film The Black Hole several chapters before it's revealed they were secretly working together. Fittingly, the reveal itself is initiated by a reference, with Harry using "Game over, man. Game over" as a signal to drop the facade.
We learn Mab has been to rock concerts and Disneyland in Cold Days, the same book in which we're hit with the line, "I was mortal once, you know." Going further back, the first time Harry met Mab in Summer Knight, he makes a comparison to Godzilla, but he can't actually tell whether she understood the reference, which, in hindsight, she likely did.
And then we have Marcone, who, when faced with one of Harry's references, opts to ignore it.
"'Elvii?'" Marcone inquired. "The plural could be Elvises, I guess," I said. "But if I say that too often, I start muttering to myself and calling things 'my precious,' so I usually go with the Latin plural." Marcone did smile that time. It was a cool expression. Tigers with full stomachs wear smiles like Marcone's when they're watching baby deer play. "Ah. I hope you can find something to suit your tastes tonight."
and
“Do you at least have a gun?” I asked. “Maybe you can distract her.” “I have a knife,” Marcone said. “Jusht like a gangshter,” I said. “Bringsh a knife to an apocalypshe fight.” Marcone gave me a level look and then said, in a much more conversational tone, “Honestly, Dresden. If you used your mind half as much as your mouth, you’d be running the place by now.”
John "Who do you think I am, I am not a charitable organization, I am a professional monster" Marcone, does not directly acknowledge the reference. He won't go as far as to pretend he's not at least passingly familiar with Gollum, or that he never watched The Untouchables. But he refuses to respond or play off of it the way one of Harry's friends would.
Doesn't change the fact that he does get the reference though.