Imagine the situation where we have a notebook, inside a tab and this tab is the one that creates the extension set and keeps a ref of the tab. (extension point). In an ideal world when the tab is removed from the container the dispose would be call, unreffing the extension set and we will be happy.
The issue is that a container calls gtk_container_remove which only call gtk_widget_destroy in the case the ref count drops to 0. Knowing this I think the extension should not care about keeping a ref to the contruct properties.