I'm not saying that you can't be in horrible pain/the worst pain you ever experienced, but can you really go up to 10? 10 for me would be passing out for the pain, crying etc... can it happen with migraines? Just because of the pain, not related neurological symptoms. it never did to me, maybe I'm just lucky.
The most painful things I can think I experienced in my life were IBS cramps and tooth pulpitis/infection, and I'm pretty sure there are more painful things to have.
Like I said, maybe I'm just super-lucky.
On the other way around I can understand why the scale starts at 1. Sometimes you can have a migraine without headache, so it makes sense.