So with the current implementation I think it just means "just show the
icon and not icon+text".

> If anything, I would like the option to *hide* labels in horizontal
> mode, as having labels makes the toolbar change size when I switch
> between windows to with different major modes.

Yes, I agree -- the intention seems to be to hide the text only when the
toolbar is vertical, but I think most people would rather prefer the
opposite if this is to be tweakable.

> So should the :vert-only property be documented?  Fixed?  Removed?

It seems useful and works, so I think it should be documented -- but the
name :vert-only is really misleading.  It should be :image-only, I
guess?