No advantage I can think of. The meds aren't THAT good, so unless there's a real fear of addiction being a problem, or side effects being a problem, the misery of the pain far outweighs any difficulty the meds can cause. And I'm not much a fan of meds (never mind what my bedside table looks like). If he's that resistant to them, he'll probably be good and stubborn about weaning off them properly, so there shouldn't be an issue. And not hurting all the time DOES help you heal, if you remember you're still actually hurt and take it slow while recovering. (Meds get you through the pain, they don't take away the problem ... like folks who medicate an injured knee or ankle and then keep playing a sport ... bad plan. But folks who medicate to get through it and actually rest will heal, and be less miserable in the process. That sort of thing.)

However, it IS his body, and cancer is the biggest scariest loss of control of one's body that I can think of, barring stuff like dementia and MS and other permanent disorders, so some folks want to have even that much control ... suffering the pain means at least they are in charge of that much. It could be how he's thinking, underneath, without even realizing it.