"The patient indicates" is still based on their completely subjective process of turning a mental state into a number. If they indicate a 9 before intervention, they did so by comparing the feeling of this this trauma to the fuzzy memory of breaking a bone as a child and the more recent but less painful memory of burning their finger in the kitchen a couple weeks ago. Then they sit and suffer from this trauma at approximately that same level of pain for the next 2 weeks, they'll have a new baseline to compare against.

It really sucks that we have no objective measure of pain; a tricorder that you could aim at someone and get a number back would be infinitely better than what we have now.

I'm saying the 9, by itself, may not be useful, but the "it was a 9 an hour ago, and now it's a 5" may be.

Not really because people have a terrible memory for pain. It can be worse afterward and still be a 5 (vs 9) just because they have got used to the pain.