Bump maps are for small, tiny, details ... pores, small hairs, moles, muscle striation, very fine wrinkles. The kind of surface variaiton that could be buffed out with sandpaper

Displacement is for midlevel details - large hairs, distended veins, knuckle and palm folds, painted on clothing, deep (age) wrinkles. 

Modeling is for large shapes - definition of form, definition of muscle and bone, very deep wrinkles

The amount of detail and clarity in your displacement maps has to do with the size of the image - nothing to do with the underlying polycount of the figure, which is a common misconception

And for completeness sake - bump and displacement maps are for surface changes in up-down (in-out) only.  Normal maps are used for more complex image driven transformations

The main issue with baked in highlights, is that there are times when they do not match the lighting of the scene and it is jarring.

"I'm also guessing though, that, if we did have those high end displacement maps... we wouldn't necessarily escape the kind of issues that burnt in effects can have... as we would probably need a selection of displacement maps for different poses or expressions."

I find that not true - morphs account for most of the changes an expression makes. Displacement details on the face are usually fairly subtle, except on aged faces where there are already morphs at work aging the structure.

"Skin wrinkles, folds and extra muscle definition added via displacement is going to be equally burnt in to each map, isn't it?" 

Yes and well thats the point isnt it? having this driven by the maps instead of the modeling makes it possible to take a figure from super buff muscle man with every vein popped out, to a skinny hairy guy with every rib showing

Part of the entire point of poser base figures is flexibility. This is why v4/m4 base shape is so bland, so the user can add their morphs and maps and transform it into something else entirely. If your base shape is superbuff, making it into anything else is a titantic struggle

Lyrra