The point of unique salts is that even if all the user's passwords are the same, the end resulting hashes that are stored in the database, are different. So, let's say you wanted to hash your set of 50,000 different, commonly used passwords. You'd have to hash that list of 50k passwords for EACH individual user. So instead of 50,000 hashes... you have 50,000 x the total number of users.