2 things...
first, the address book does serve a purpose, you start typing someone's name in the TO: of CC: or BCC: field and mail.app searches the address book database and throws up a list of names...don't cut and paste, thats freakin retarded...the address book also stores a temporary file of people you have emailed recently or who have emailed you. This saves a lot of time so you don't have to go and type all that crap in...akhm akhmm

the dragging and dropping would be nice, but personally i have no desire to leave my address book open, if you can use the keyboard to do something it is typically a lot faster...
second, I have a feeling there have been a lot of changes or will be a lot of changes to how the address book is integrated with the system as well as how the mail.app program functions. There have been some somewhat large changes to what is on apples page now about mail.app and how the public beta mail.app functions. Check out my thread in the General discussion for more info and links