When you talk about pain being good, you're talking about the information it sends being useful to survival, not about the method of signalling (pain).

Just as you looked at CIPA patients to ask what's good about pain by looking at those who don't have it, you can look at people who suffer from chronic pain to look at what's bad about it.

People with chronic pain have the method all the time without the useful information, and their lives suck. Chronic pain suffers are exhausted and depressed because they're fundamentally unable to do anything without it hurting.

Worse, because people without chronic pain don't highly dis-value chronic pain, it's not respected as being as bad as it is -- most people, when asked, would prefer chronic joint pain to a broken arm, yet most people with one of these conditions have the opposite preference, for good reason.