Well not useless! Just suppose this however:
Im assuming passwords in cookies are encrypted so that if someone hacked your computer they could not gain access to it...
However, the hacker can simply copy+paste the cookie file over to their system and login to your account...
It prevents them seeing your passwords if you use the same one, but it does not prevent the from entering the system...
A solution? How about storing an encrypted IP address in the cookie, and the cookie is only valid if the IP address matches.... However this wont really work for those 56k people....