You can doing spacing in the layout
I just thought it would be easier to define a simple item spacing, instead of a workaround. I'm using lots of listboxes, so this would save memory, time and extra definitions.
Is there a really good reason not to have this feature?
I'm not sure what you are asking here, if you are creating an item renderer and changing the layout(modifying the default gap of 0) of the list it is no longer a 'simple' list. Whether you chose to or not lists use layout as part of the design, the default simple list sets the layout to vertical with a gap of 0, like any programming you override the default to customise a component.
Overriding default behaviour is not a work around..
If you have 1 or a thousand lists that have a non standard gap but want consistency across the application you make a copy of the default skin change the gap value then use your skinclass on all your lists. You can go one step further by extending the standard list with a default of your skin then use that component throughout your application.