In EMS, we use the 1-10 scale for pain (where a 10 is the worst pain you've ever felt in your life). Yeah, for middle numbers it doesn't hold a lot of meaning, since folks' pain tolerance varies quite a bit. But for trending a patient over the time that you are working with them, it can be very helpful.
For example, if I'm called to assess a patient who is complaining of chest pain, I'll ask how long ago it started, and on the 1-10 scale, how much did it hurt then, and how much does it hurt now. I can ask the same question later in the assessment, to see if things are getting worse or holding steady for a while...
And usually, when a reliable patient says the pain is an 8 or a 9, that's a pretty big flag that things may be serious...