This is a good argument for unique passwords, not for weak passwords. Weak passwords only "make sense" if you really don't care whether your account is compromised due to a very weak password.
But often you don't care. The value of a throwaway account you made to download a file is practically zero to you or an attacker. In the tradeoff in simplicity (all my crappy throwaway accounts have the password 12345... easy!) against security, simplicity wins.
If I only made an account on one of Cupid media's sites because I wanted to see a picture, I wouldn't care whether my password was easily guessable. Additionally, I'm fairly sure that an easy to make account with no access privileges is completely worthless to an attacker as well, and so the likelihood of anyone even attempting to compromise it is next to nothing.
This is 100% true in my case. If I value the service, it gets a unique password. If it's Gawker Media, well here's a hash for 12345 and have fun with it.
