In the diagram view for a schema, it helps to have a visual cue about
when an element is optional, e.g. a dotted line to distinguish
it from mandatory elements. Similarly an indication of the occurrence bounds
on the diagram is also helpful. It's true that these are visible on
the properties window, but only when the element is selected. This does not
give as wide a perspective.
The practice is common in UML-style diagrams and is very useful. It's
implemented in XML-Spy