I have used two poles for hundreds of kilometers of hiking and my knees really notice it when I forget to bring them along especially when hauling a heavy pack over mucky terrain or covering a lot of vertical.

The only place where I don't like poles are on steep scrambles where two free hands are more useful that a couple of pointy sticks.

I use Black Diamond poles that have a "Flick Lock" system to lock the pole length and it allows me to quickly shorten the poles for the uphill and lengthen them on the down hill. The Flick Lock can be adjusted with mitts on and have worked well in the winter where I have seen other poles freeze up.

I prefer a three section poles as compact a little smaller and can be strapped to a pack when not needed.

Pole shafts are also a great place to store some extra wraps of tape.