There are other reasons that other ingredients are added in RX pain medications.

1) as a potential discouragement to abuse. However, most determined people ignore this.
Also to extend patent coverage by the manufacturer. The addition of acetaminophen to Tramadol is an example of this. This gives the manufacturer a longer time interval to make money because the mixture is patented...so that patent has to expire before generics can come out.

2) Acetaminophen does not act on mu receptors like the opiates do. This drug has a long history:
http://en.wikipedia.org/wiki/Paracetamol
not mentioned in this article is acetanilide:
http://en.wikipedia.org/wiki/Acetanilide
This was called Dr. Miles' pain pills in the past. Miles labs grew from that, and was bought by Bayer ultimately. Acetanilide was found to be toxic, but acetaminophen was derived from it. The risk of toxicity remains, but is less ...unless you overconsume it.

We know today that acetaminophen does have some anti-inflammatory actions, so that is useful, even though it is a mild effect. Also it has been added to the drug list for patients to avoid who use coumadin. Considering it has been around for so long, it sure took decades to learn more about it. I still cannot find an explanation of "how it works" on pain though!
This link from the official website is still vague on mechanism of action:
http://www.tylenolprofessional.com/pharmacology.html
If a person has liver damage from hepatitis, or cancer, or other reasons, then the 4000mg ceiling for acetaminophen becomes much lower. 2000-2500mg is often mentioned, and often suggested as a daily ceiling for most people today, but it is not widely accepted.

The big problem with it is that there are so many mixtures on the
market with it in it. Tylenol PM, Excedrin migraine and most of the cold/cough mixtures. One has to be very careful with OTC products today in general, as they can have confusing ingredients.

Acetaminophen has been around for a long time...its parents were very toxic. Phenacetin was just taken off the market in other countries around late 70's early 1980's or so. The old APC's sold around the world were aspirin, phenacetin, caffeine. They were more common in Australia and Europe, and the phenacetin caused kidney cancer. Acetanilide was replaced by phenacetin, which was replaced by acetaminophen...but the whole family is one with potential toxicity.

The Percocet content of 325mg acetaminophen is quite negligible as a pain reliever. It is in there mostly for historical reasons, as the patent is quite old. The other strengths were added much more recently. Percocet comes in many combinations:

from http://www.rxlist.com/percocet-drug.htm

For a long time the oxycodone plain immediate release tablets were being sold without FDA clearance...and there was a shortage of those a few years back when the FDA demanded new drug applications for them. (the same thing happened with some morphine formulations) Percocet is an old drug...in fact it was the second oxycodone containing drug FDA approved and for many years was the only form. It went thru the channels for FDA approval properly. The first was from the same company and was called Percodan...and had aspirin instead of acetaminophen.
But in the late 70's acetaminophen started being mixed with codeine, and Percocet closely followed. Previously codeine was mixed with apc's...and was called Empirin #2,3,4.
There was also Phenaphen 2, 3, 4... that had phenacetin in it as well and it was the competition for Empirin's. Those had to be reformulated and Empirin /codeine, today has no caffeine or phenacetin.

There are still dentists today prescribing Percodan and Empirin with codeine. They prefer the aspirin for the anti-swelling effects.