If everything is correct, they don't "need" one to prevent head shake under normal conditions, but they do improve ride quality by dampening out feedback, and wandering on uneven surfaces. They can also reduce the abruptness of perceived yaw under throttle, and braking. Nice to have, Vs "need" to have. The only time my Urals have exhibited any head shake is when something was worn, loose, or out of adjustment.
Strangely enough.... one of my sidecar that I mounted needed one; DMC re-mounted it with a subframe (Vmax) and I do not need one anymore!!
Much of the steering damper debate centres on the head shake issue, usually allied to suggested incorrect alignment. The prime reason for my preference relates to external factors that can affect control. White lining, truck grooves and road debris are indeterminate factors that can cause problems. As an observation, I note that some of the examples posted have one end attached to fork lowers. Might I suggest fixing to a solid part of the front forks.
I know little of this subject. Just commenting about my experience. When I installed the Escort on my Valkyrie using a DMC subframe I experience no head shake unless I removed my hands from the handlebars. Then, at any speed slow or fast I got immediate tank slappers that were scary and difficult to stop. When I put on a new front tire all that went away. I could ride any speed without hands, not that I did that often. Now that the new front tire has a few thousand miles on it the no hands head shake has returned a little bit but not to the original extent. Yet.
I typically prefer just enough dampening to soften feedback from such surfaces, but one place I find more dampening can be really nice is on rain grooved freeways, especially with sidecar specific tires like the K28. It reduces, or eliminates tracking on them.
I keep my damper on quite light unless the conditions require more. I have found the conditions that require more is rough roads. Off road also requires a bit more.
I already showed the one on my 1200C (on the previous page), the one on my other bike is the stock BMW damper found on a /2.
Hmm. Interesting, especially with the adjustment feature. First steering damper on our first serious outfit, early Honda 750 four with Palma, was originally off an old Matchless bike. Maybe it's back to the drawing board.