I think you're partly right; in my experience, polypharmacy is more common than an intelligent use of more than one opioid. However, methadone also is an antagonist at the NMDA receptors and is more effective for neuropathic pain.
But the prescribing of oxycodone has gone up 400% in the US in the past ten years. Americans are the #1 users of opioids worldwide. Were there that many people in severe intractable pain B4?
Back O/T: as a methadone dispenser, I'm always phoning drs re: drug interactions. Methadone drs are fairly up on the interactions, but other MDs the pts see may not be. Methadone is a substrate for CP450 3A4, which is huge, so tons of drugs affect it, either raising or lowering its levels.
Recently I contacted public health about a methadone client started on rifampin. The guidelines where I work suggest methadone patients receive rifabutin instead, but it's not covered by Drug Benefit (like Medicaid), so they kept her on the rifampin, which potentiates the metabolism of the methadone. So...this lady experienced WD symptoms, and has relapsed into street drug use.