Diceware encourages reuse of a single password on multiple sites. It's also much more time consuming than just clicking the "randomly generate password" button and having it spit out a 30 char password. That, and a randomly generated password of, say, 16 chars is much much more secure than a diceware generated password of 4 or 5 words, IIRC. And I'd still need a password manager, because reusing passwords on different sites / accounts is a big no-no.