Acetaminophen in particular is somewhat scary. Just 2.5x the recommended maximal daily dose of 4g can lead to complete liver failure. With alcohol, other medication, or existing damage, much less could be enough.
At the time, it apparently seemed a good alternative to aspirin which does cause some number of people to have stomach problems. (As well as having a particularly extensive repertoire of only somewhat understood physiological effects, for better or worse.)
If true, it's alarming that the way to "prevent abuse" is to poison the patient.
Of course, addicts have found ways to extract the opiates from the adulterants, so it's not even effective.
Adding it for synergism makes sense and is what I was taught in medical school. I doubt they add it to deter abusers, but I could be wrong...
Here's an interesting discussion from 2010, before they were rescheduled
https://painmedicine.oxfordjournals.org/content/11/3/379.ful...
I was told by an emergency room nurse that Vicodin is not killing patients/abusers; it the liver damage that results from the acetaminophen.
A few years ago, I heard they were talking about removing it, but lost track of the outcome.