Redundant design elements often sit around for the life of an application since it's difficult to be certain that a design element is no longer used.
It would be extremely useful to have a tool or special design view showing reference elements that are not referenced anywhere (eg subforms, script libraries, hidden (agent list) agents, style sheets etc.
Some believe that unused views cause performance degradation but actually by default, a view that has not been accessed in 45 days loses it's index and has no server impact. Therefore this is about keeping things tidy.
Going a bit further, an opposite tool / view that shows referenced elements by hierarchy, a visual representation if you will, of part of an application's design structure.
This might be achieved with DXL but I'm not certain.