I don't do "Nordic Walking" exactly. However, I do like to use a single trekking pole when I hike. Many of our local hikes are on steep and rough trails. (Indeed, calling some of them "trails" is stretching it!) I find by using one pole it is easier to maintain balance. On rough ground you expend a lot of energy just keeping and/or recovering balance, especially when coming down. A pole is also handy for balance when stepping from rock to rock to cross a stream.

I seem to get most of the benefit from one pole, and that keeps my other hand free. I like the adjustable poles, since I can change the length to suit the terrain, and when on flat easy ground I can collapse the pole and attach it to my pack. I personally don't see any benefit in the shock absorber models.

I find by using a pole, at the end of the hike I am less fatigued and my aging knees feel better.