Good question.
The distinction here is that the damping force is velocity dependent, the trail force is not. That is, when you apply more force to the bars as you try to turn faster, the larger the resisting force supplied by the damper. For slow steering corrections there's little effect as you point out. As you crank up the damper, you get a larger resisting-force from the damper for a given rate of angular rotation of the handle bars. So in fast side-to-side transitions, increased damping slows the rider's handlebar input. The faster the transition or amount of damping you dial-in, the greater the effect. Indeed, that's why you have a steering damper, to resist fast-acting tank-slappers. Adjustable dampers allow you to strike a compromise between instability and sluggish steering.
Dynamic stability is a complicated issue. Flight vehicles can be described analytically in terms of stability derivitives but motorcycles are just too difficult to simulate on a computer. So you do testing on new designs instead. Somewhere along the way, Ducati engineers and test riders found a need for a steering damper and the need to warn you to use the steep steering angle with caution. You may never encounter the riding condition that they did, but I suggest that you be ready to deal with it.
BTW, I studied aerospace dynamics at university and worked for decades in the field of structural dynamics, so stability analysis is my field of expertise. When I suggest caution here, I do so simply because it’s a true safety issue.
[Edited on 6-18-2004 by Shazaam!]
Shazaam!
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