In the past, I and others have produced many excellent examples of why a modern, high-end CAD system should be able to handle ZTG. 

All other reasons aside, the simple fact is that many come from importing files from other CAD system that can do this.  Not being able to cleanly import or manipulate this CAD data leaves the tree with a mess of errors.

Some people say that ZTG is a physical impossibility (bunk), but here is a good example I ran across when importing a file.

When creating a sheet metal part with a punch/tear like this, there is a ZTG condition in real life.  We can argue all day about the molecules if they are touching or not, but these are just silly arguments.  The fact remains, the attached part is just ONE MORE example of legitimate, real-life parts that need ZTG capability.