I am not completely a trekking pole advocate, but there are times when they truly help. Of course the better one's physical fitness, the less poles are needed. And for those with old injuries or joint problems, they are probably necessary.
Wisner and I recently did a long hike, and on day 1, we climbed 9,000+ feet in 14 miles. Craig really did not need poles, but they sure helped me on some steep sections (I am about 25 years older than him). I would guess that poles would benefit at least 90% of BPL members on this trail.
On the morning of day 2, my arms were a little sore, but my legs felt great. So I attribute that to the poles. On day 2 we did 26 miles. At about mile 20, we had a significant climb up some seemingly never-ending switchbacks. The poles helped me, especially since I had injured my foot the day before. Craig probably didn't need poles. But keep in mind, Craig does a huge amount of running to include some 50K races. Most of use are not in that kind of shape.
Last day my arms and leg muscles were fine; no soreness at all. We descended 6,000 feet in 11 miles, in the heat of the desert, on some very rocky trails. The poles helped in some sections, but were not essential. But my legs probably appreciated them the days following the hike, as I had no soreness.
During this hike, we did several miles on steep, snow covered slopes, where the snow was at least 3' deep in sections. The poles helped both of us.
Both of use took trekking poles on this trip. Craig often carried his on the outside of his pack. I always carried mine in my hands. Since these are LT4's and very light, I frequently just carried them parrallel to the ground, or just used them lightly, in cadence to my steps, not to propel me on the flat sections. Sure, not the most productive way to use them, but I didn't feel like attaching them to my pack. On these sections, the clickity-clack of the poles probably annoyed him :( — although he did not complain.
So, I think poles are appropriate when physical fitness, terrain, and elevation are factored in. Not essential, but helpful in certain situations. For some people they are probably essential in any situation.
I almost never take trekking poles on day hikes, even if I am doing close to 20 miles… extreme elevations might be the exception. I often take a hiking staff out of habit. The staff is an old friend of over 20 years. Its main function is to move rattlesnakes off the trail, and it is nice to lean on when taking in great panoramic views. Plus the staff compliments my almost white beard, for that mountain man look :)
So again, if you think they help you, use them. If you do not need them, you just saved some money and weight. But we cannot say they are not useful for some people.