Usually you will first lay out the basic movement of an animation, meaning you will have set up maybe 6 or 8 basic poses scattered through the timeline. In a running animation, for instance, the basic poses will be the character with both his legs on the ground taking impulse for a step, then jumping forward with one of his legs stretched out, then landing on the foot of that leg, etc. If the animation feels awkward, you'll adjust these few basic poses and the timing between them until the overall motion makes sense and feels good. It's great, it's just 6 or 8 frames to keep track.
All good so far! But then you need to start adding keyframes all around the timeline to add micro-adjustments (like aligning the char feet in frames where the interpolation doesn't produce the desired results) and secondary movements, cause we all love characters with bouncy hair and clothes and weapons! The thing is, the timeline then gets really messy, and it becomes much more difficult to then go back and tweak the overall movement if you ever need to do that (you usually do, as you guys know animation is all about iteration and refining). So when you're about to start adding secondary motion, it really feels like a leap of faith moment, as once your timeline gets keyframed at every single frame, it's practically impossible to keep track of the 'main' keyframes.
So this is how I imagine a dopesheet layer for secondary movement could work: we have a button that we press to let Spine know that we're now adding secondary movement. When we press it, all new adjustments in the dopesheet will be done in a separate 'layer' that can then be turned off when we need to back to tweak the general motion. This second layer would always be applied on top of the main movement, and its keyframes would be intertwined with the ones that define the main movement.
Wouldn't that be useful?