People queue to quote Bruce Schneier saying:
Okay, somebody steals your thumbprint.... Because we've centralized all the functions, the thief can tap your credit, open your medical records, start your car, any number of things. Now what do you do? With a credit card the bank can issue you a new card with a new number. But this is your thumb--you can't get a new one.
This is a best illustration of the need for meaningless primary keys in databases that I've seen. If you ever have to design a database, or anything.
