With massive compromises of plaintext passwords, attackers now have a growing source of wordlists derived from actual usage. Not only can you add the most common passwords to a wordlist, but you can even sort them in decreasing order of frequency. An astute attacker could even apply machine learning techniques like clustering and classification to determine which other words are missing. This could be used to identify popular memes (such as Korean pop stars), and lead to new words that are likely to be used in the future.

Hashed passwords posted after compromises are increasing attacker knowledge as well. Sure, your password hasn’t immediately been exposed but it remains available to anyone with the right wordlist or enough computing power, forever. As more of these are cracked, the global picture gets clearer, and you may be vulnerable to a targeted attack long after the original site is gone.