The funny thing is, try to explain this to your doctor when she wants to prescribe an opiate like oxycodone.
In about half the cases I've been prescribed opiates the doctor refused to prescribe oxycodone on its own -- I was told it was Percocet (oxycodone + acetaminophen) or nothing, they would not write a prescription for just oxycodone. I had one surgeon do it reluctantly, pointedly asking me why and not really liking my answer that I felt it was dangerous and could add in acetaminophen on my own if I felt it was helpful.
I did have one specialist who wrote that way and when I asked her why she prescribed that way she said current research showed the liver risk outweighed the small benefits. Ironically she was the "less educated" physicians assistant and not a full MD.
I think most doctors believe its beneficial but I also think they somehow see acetaminophen opiate formulations as some kind of bulwark against abuse. Either because they believe it is so much more effective paired with acetaminophen and you'll be inclined to take less overall or that people "know" acetaminophen is bad in quantity and it will serve as a deterrent to excessive dosage, especially people with a history of drug abuse.
I also think they are highly skeptical of someone asking for a specific opiate formulation, even when they initiate the prescription (ie, you have an obvious injury and they prescribe an opiate). It's highly ironic that they're so worried about addiction they're willing to risk serious liver toxicity.