As I understand it, the problem is that many people use the same password on every forum. When they do this, an unscrupulous webmaster can use the hash to login

As I understand it, the problem is that many people use the same password on every forum. When they do this, an unscrupulous webmaster can use the hash to login as these members on other forums they post at. The double-hash in vB eliminates that potential issue and adds another layer of protection to forum members. ...