Something that seems to be getting ignored in the discussion of pain being good is the existence of pain asymbolia. People with pain asymbolia still get the signal of pain, so they know about damage and can mediate it, but it doesn't feel bad. If we accept that having the information content of pain without the negative affect would be preferable to having the information and the negative affect, then there's clearly something bad about pain.

I think there are two main bad things about pain.
1: Pain produces a strong negative affect, aka suffering, aka I just hate it.
2: Pain produces an aversion strong enough that people will do anything to stop or avoid enough of it. That makes pain something that can severely restrict people's freedom just by existing, more so than most other unpleasant things.