I'll throw in my own story, for what it's worth...
I just finished hiking 2/3rds of the Colorado Trail, from Denver to Creede, CO (350 miles, 28 days). Very different from the Camino, details of which I may post on another thread somewhere.
But as to poles, while I was doing my "shakedown hikes" before the actual trip in the mountains around my home, I found my knees hurting badly, especially on the downhills. I was very worried about how they would feel on the main trip and so I grabbed a friend's set of hiking poles before I left- no knee problems on the Colorado Trail at all!
Now the CT is MUCH more mountainous than the Camino and I found myself stashing the poles in my pack on the few days the trail was flat, but I think the poles saved my knees.
My main dislike of the poles is that instead of thinking about where on the trail to place two appendages, you now have to worry about four.
In summary, I'd leave the poles home unless you have a history of knee problems. I also love the previous idea of bringing one pair for two people to share, as needed.
-from American Pole (John Wontrobski)