Just did an adventure race across the Grand Canyon (R2R2R- 48 miles in one day) and hiking poles seriously saved me. I've said this before but it is estimated that they let you go at least 1/3rd longer (if used effectively). I don't remember the source, but some of the reasoning includes:
-The weight of your arms alone is at least 5% of your total body weight. So 2 arms equals 10%. If you are using poles to at least rest your arms on, you are taking that weight off of your total load on your legs.
-if you actually push off on the poles (especially uphill) it takes even more load off of your legs
-Poles help your arms do more balancing and steadying on rough terrain while legs do more power work. This helps to not wear out the less-used muscles in your legs
-During descents, you can use poles to lower yourself thus reducing pounding on the knees and the working of the quads.
-Poles are also great for stream crossings

In this particular instance, I pulled a calf and had some serious problems. I don't know how I would have finished without being able to transfer a lot of my weight to my arms through the use of the poles. That being said, it sure does give your arms and shoulders a workout that most don't get while hiking!