DWorks, you should give stories/ storeys a try. I think you'll find they are significantly more useful than the old Z/delta-Z system.
For one thing, you no longer have to manage heights manually (as you said).
For another, many more objects can be set to automatically get their height (in Vectorworks 2011 and earlier, only walls weould change):
-walls
-columns
-pilasters
-stairs
-escalators
-window walls
Lastly, a Stor(e)y definition can control any number of layers. I'm not sure why you say "the storeys will only become usefull if you actually can place objects on a storey, and not on a design layer." This seems to me to be just a technical distinction.