I am fascinated with trail running. However, I am reluctant to start doing that regularly because of the descents: from what I heard (and from limited personal experience), you need walking poles for steep descents with many rocks and/or roots.
But why do experienced trail runners never use these poles? Is it because of the additional weight, or maybe they want to keep hands free to stop a possible fall?
How do they compensate for the increased stress on the knees during descents?
There are plenty of trail runners who do use poles. They do give you an advantage so some races ban them. If they are banned in a race, training with poles could well be a disadvantage, at least if you always train with them.
Some examples showing the practice:
Personal Experience
I have run with poles occasionally, and find that it changes how the run feels. It can feel physically easier, but occasionally mentally harder. It can take a lot of co-ordination when you're trying to make sure you place your feet and the tips of your poles in the optimal position when crossing rougher terrain. I'm sure that gets easier with practice and when you're fresh can be part of the fun. Having an extra push on the uphills can also be a benefit.
Erosion
Also remember that poles can cause more erosion then shoes, and the tiny hole they create can expand and cause the surface to start running off if there is lots of water around. There was an article about this in one of the 2011 issues of The Scottish Mountaineer but I can't find a link the article. Here is a link to a discussion about the article: http://www.mountainactive.co.uk/blog/2011/05/25/walking-pole-use/
Stress on knees
Regarding your point about extra stress on the knees, based on my experience poles make a difference in this regard when walking but not when running. When running poles tend to be longer and mostly behind you (like in cross-country skiing) which won't help so much on the decent. I personally find that moving fluidly over terrain helps mitigate against the extra stress. In fact, I find walking down a hill much more stressful on my knees than running (or what feels like floating) down.
If the extra stress on your knees is a problem, strengthening the muscles around the knees may well be helpful. Bulgarian Squats, also known as split squats, are particularly helpful in this regard: https://www.verywellfit.com/how-to-do-a-bulgarian-split-squat-4589307.
I do a fair amount of mountain trail running in Southern California, and I don't use poles. This whole idea of stress on your knees is something that I think just exists in people's heads, or maybe there are some people who have vulnerable knees. I've had my share of running injuries, but never the slightest twinge in my knees. I do worry about taking a spill on the downhills, but I doubt that poles would help in that situation. Besides the fact that I see poles as adding no value whatsoever, they also turn the activity into something else. I want to go for a run, not a hike. The allure of the activity, for me, is its simplicity. Just me in shorts and a T-shirt and a fanny pack, nothing else. Poles also decrease efficiency.
Having said all that, I think there definitely are some trail runners who use poles. This seems to mostly be a thing among mountain ultramarathoners.
