The "default" passwords on any network device are there to allow anyone to easily setup their Router or Modem or Hardware Firewall.


The problem lies in that most never change them to something more secure AFTER setup.
You should never leave the defaults in place-- if you lose your new password (which you WON'T -- if you write it in a safe place)... you can always reset the device manaully.
I actually use Roboform (with a password lock) to remember mine-- and I actually periodically backup to disc AND print out a hard copy.

Leaving the defaults is an open invitation to:
UNauthorized Internet Connection use (wireless) Wardriving
Identity theft
all manner of other nasty consequences...

First case of "drive-by pharming" identified in the wild - Network World

and the above was AT A BANK-- so it is a big problem in some cases.
Even home users should protect themselves