The behaviour for the default value of a property is actually working a bit different than you describe.
The default value is a value that will be set for the property when the page first is created, it is not a value that will be used if the property value is empty.
Hope that makes sense.
I created the property "FooterText" on my standard page: String (<=255), and put the default value. But it doesn't work correctly: the default value isn't appeared in the site.
<EPiServer:Property PropertyName="FooterText" DisplayMissingMessage="false" EnableViewState="false" runat="server" /> - is empty.
Value must be entered - unchecked
Searchable property - unchecked
Unique value per language - checked
Display in Edit Mode - checked
Can anyone expline this situation?