This would let us (and our clients) treat our AFD-derived classes in a more generic way. The my search branch for bug #646808 made this usefulness obvious (and I'm cherry-picking it for this bug to get it merged before that branch).

For example, one could handle all AFD-derived classes without needing to switch between their specific types as in:

if (FOLKS_IS_ABSTRACT_FIELD_DETAILS (fd))
{
  GType value_type = folks_abstract_field_details_get_value_type
(FOLKS_ABSTRACT_FIELD_DETAILS (fd));
  if (value_type == G_TYPE_STRING)
    handle_afd_str (fd);
  else if (value_type == FOLKS_TYPE_POSTAL_ADDRESS)
    handle_afd_postal_address (fd);
  else
    ...
}

This is particularly useful for the first case; most AFD-derived types have value_type G_TYPE_STRING, so this saves a bit of repetitive code. It also helps future-proof the code a bit (since it's possible we'll add more AFD<string> classes in the future.