Re: So Yahoo! hack = No big deal for me.
Maybe not immediately.
But when the people in your address book have all been profiled and identified, there will initially be a missing jigsaw piece (you).
Probably easily filled by "triangulating" the data supplied by these other people which inadvertently identifies you.
Maybe correspondent "A" (in your address book) emailed correspondent "B" (also in your address book) about something you were also involved with, where personal information about you is given.