How many members of the public think that they have to use their E-mail account password as their password for Apple ID and every other amateur-hour site that enforces this dumb rule?
MILLIONS. I would bet a decent amount of money on it. So if any one of these sites is hacked and the user database is compromised, all of the user's Web log-ins that have this policy are wide open.
Then there's the simple fact that everyone's E-mail address is on thousands of spammers' lists. A simple brute-force attack using the top 100 passwords is also going to yield quite a trove, I'd imagine.
Apple IDs didn't originally have to be E-mail addresses. They're going backward.