You might think it's obscure, but because WC asked you to change your password means that it is already being used by several others. Therefore it's really not that obscure.

(Speaking from my experiences at Elftown)You might think it would be your problem, however people tend to pester the guards a lot saying they have forgotten their password and didn't add an email address, or someone has 'hacked' their account and can no longer log in, this becomes our problem and we are the ones who have to deal with it.

The system was implemented to encourage users to use passwords that are more complicated and less easy to guess. Many large communities encourage users to use more complex passwords to increase security, ideally a mixture of letters and numbers.