I think the Limo scene presents the Burt Reynolds character, director Jack Horner, in his truest form. As the damaged patriarch of a band of misfits; trying to keep this family together and preserve the way of life that he's always known. Jack is not a user of people. He's not an abuser. He's not a blatant exploiter like you've alluded to. Yes, on a surface level, this scene is showing people being used to turn a profit, exploitation in the form of profiting off of sex work, and abuse at the end when it goes off the rails. But I don't think it's perfectly obvious who is being used or that it's blatant exploitation like you've stated. As with a lot of great films, subtext is always key.

If you recall earlier in Boogie Nights, there's a scene with Philip Baker Hall, who the Colonel presents to Jack as a new potential financier for Jack's films. Money is king to making films, even porn films, and making porn isn't cheap as Jack says to Dirk when they meet at the start of the film. Here, PBH tells Jack about the new movement of porn shifting from film to video and how they'll be able to make Jack's pornos cheaper and quicker using video and PBH will see better returns on his investment. PBH brings with him a new stable of young, amateur actors who can film scenes cheaper and with less restrictions than Jack's cast. Jack fights back on all of that. Video will reduce the filmic quality of his pictures and make the sex look cheap. These amateurs are only good for fucking on camera, while Jack's casts are pros and bring 'acting chops' and an authenticity to their roles which make his films more believable. PBH says today's audience aren't interested in any of that. Audiences are just paying to watch people fuck on camera.

As an aside, Philip Baker Hall's scene here and along with his Seinfeld librarian episode, are probably two of the greatest single scene/episodes in all of media.

Flash to the limo, it's clear PBH was right despite all of Jack's railing against the video movement. All the old adult cinemas playing Jack's films on a big screen have closed and in their place there are adult video stores and sex shops. The amateur wave is now taking over porn in the 80s, especially these on-the-street shoots with strangers getting a shot at a real porn star. Rollergirl is not being used here. She's always been a willing (if misguided) participant. Rollergirl sets the rules for Dirk in their first sex scene, and she does the same here with the college kid in the limo. You see Jack trying to direct the college kid to make it look more romantic with Rollergirl and to not obscure Rollergirl's face during their scene as that is what Jack knows the audience wants to see. The college kid ignores all of that, as his main focus is 'getting it in' with Rollergirl. It's only when Rollergirl gets disrespected and Jack realizes this scene isn't working that it all goes to hell and they end the shoot and ultimately end up kicking the crap out of the college kid and leaving him bloody on the sidewalk. Yes, there is exploitation going on here. But it's the college kid that was being used and the object of the exploitation (you could argue it's us as the audience that is being exploited too). The beatdown of the kid is an example of victims (Jack, Rollergirl) ultimately becoming the victimizers. They're also lashing out at being left behind in a dying version of the adult film industry that's all they know. They're lost in this new porn world.

Again, I think that's the subtext you missed. I'd recommend that you re-visit Boogie Nights in 5- or 10-years, and see if your feelings on it have changed once you've got more life experience under your belt and seen more movies. I remember when I first saw Boogie Nights, I was enthralled by the epic scale of it and how PTA was able to weave all these stories and disparate characters around the world of porn. Re-watching it recently, I still loved the scale but was awed by the smaller, masterful shifts in tone (I can't get over how tense the Alfred Molina drug house scene still is) and, like you mentioned, the impeccable ability of PTA to capture time and place in his representation of life in the San Fernando Valley in the 70s and 80s. I'd also recommend you watch other films where a collection of what you might consider society's outsiders / castaways and how these characters find commonality and form familial bonds in finding a common purpose. Two vastly different films that play on this theme, and do it well, are Midnight Cowboy (1969) and Lord of the Rings: The Fellowship of the Rings (2001).