Referring to this post: http://drupal.org/node/574400#comment-2029904 :
Default values are not applied during "Add more items" operation. [...] If you want to apply default values of all "Add more items" operations, then you have to do it yourself starting from hook_form_alter() ...

This may make sense when using CONTENT_CALLBACK_DEFAULT so that the content module handles default values on its own.

When using CONTENT_CALLBACK_CUSTOM, meaning to implement hook_default_value to provide default values, the developer has a special purpose on setting default values. It would be nice to give hook_default_value a call every time when a new item is added to the form.

Example:
Think of forms with lots of items, where the amount is what counts, not the individual data because its added using rules. In my case I need to add checkboxes to the form (I'm developer of http://drupal.org/project/cck_inputs ), representing seats in a theater. A theater has tons of seats identified by a seatnumber. Therefore, I can number the seats with default values when they are added to the node.