What WOULD reflect the pain they feel? My own arbitrary scale pegs 10 as the worst pain imaginable, and I can imagine some pretty terrible pain. If I'm not given any frame of reference, the highest I'd rate any pain I've been in, from horrible toothaches to broken bones, no higher than, like, 3. At least this has something tying that number to something else.

I see a number scale like this only really useful for tracking an individual as they progress. In which case, I'd say it'd probably be best to start at 5, "okay sir/madam, right now whatever pain you're feeling, were going to call that a 5. As we progress through your treatment, I want you to remember this pain, as 5, and tell me at given times if it's better, or worse, or the same on a scale of 1-10."

Assessing initial pain is a lot trickier, precisely because you don't have an agreed upon scale. Also, people are WILDLY different with regards to pain. I have a really high pain tolerance, as long as I know why I'm in pain. The thing that bothers me more is not knowing what's causing a pain, Whether it's actively killing me or just trying to make me miserable. Should I include that worry in my pain scale? Or use whatever scale I want to? Or just agree upon a scale and run with it?