I was just today re-visiting my confusion on this issue. I think I finally understand that the Xowiki instance 'Xinha WidgetSpec' parameter only configures Xinha for the forms used to create or edit xowiki objects.
I now finally understand that adding a xinha config to a formfield spec is the only way currently to configure the editor for Xowiki.Form fields.
In my current experiment, where I have several textarea fields that all need xinha activated with the same configuration, this is very verbose and repetitive.
It would be great to have the ability to set a default spec for xinha for use when filling-out/ creating instances of Xowiki.Forms.
regards
Richard