These modifications allow you to assign specific address book entries to a default shipping and a default billing address. In my circumstances, I import osc data into an accounting software package that allows for a statement address (customers_default_address_id), a billing address (customers_billto_address_id), and a shipto address (customers_shipto_address_id). Along with this, it would be easier for the customer to keep track of a "nickname" for that address id (also nicer in adrop-down listing for selection purposes when checking out! :)
There is an additional address line as well as a telephone & fax for each address book entry. Useful when selecting 2nd day air or overnight shipping options and the carrier requires a phone number for the delivery address.
Also, I modified the way osc handled updating customer account along with changing customer name upon selecting a different primary address. I don't see the need to do this. Why would the customer's name change if you wanted to use a different primary address? This particularly bothers me when using the address_book firstname & lastnamefor a contact or ATTN: field.