I think the default address book should contain a reference to all contacts in other address books, or at least have an alternative global address book with the option to make it the default. Not only would you be able to find a contact easier if you don't remember what address book they are in, but mobile users using activesync would have access to all their contacts.
My current workaround is a script I wrote this morning. I haven't tested it thoroughly, but it seems to work. For all users listed in /etc/zcsync.conf, it checks all their address books (including links to address books shared by other users) for new contacts and copies them into the "Contacts" folder. Having a global address book would work much better, though.