If you use the same password at every website, however long and complex it may be, then if a hacker manages to work out your password at one site they will be able to use it for all your websites. You may not be too worried if a hacker discovers your facebook or twitter password, but what if they can then use the same password to access your paypal, ebay, amazon or other online banking and e-commerce sites? What if they can use the same password to access your personal email? Or use web access to your work email and steal confidential information from your employer?
Hackers stealing passwords is not uncommon and there have been a number of high profile cases of entire password databases being stolen. Research into those databases also shows that (a) a significant number of people use simple, easy-to guess passwords, and (b) a significant number of people use the same password on multiple sites.
