I agree that thumbnails are handy and would be lovely to have.
I have just to add something to it. I do prefer to have it for each workfile from simple reasons…it should be useful not for show of. So it should help artist to locate the proper workfile with visual aid and thats it. The only purpose.
So Im against single thumbnail for shot only (from latest or selected version) its just bells and whistles not being really too much of a use.
Here is how it is in blender and can tremendously help to find the right workfile visually (not needing to read the names) We should strive for this. Could be just viewport capture of active viewport. So artist would be aware that the workfile saves with whatever he has on the screen. So he could benefit from it and set it to his liking communicating the content of it for later search.
This screenshot just shows how well it communicates the content of the particular workfile. Not speaking of the layout.
I like the mockup one @BigRoy made…thumbnail being on the right “details” pane is pretty fine.