The reason so many people used the deleted items folder as a filing system is exchange didn't set a quota on that folder and by default would only get emptied by a manual emptying. So back in the days when a user could have a 100mb mailbox quota 'unlimited' space could start looking very attractive. Especially if that quota was 5 years old because the server hadn't grown with the times, since what business wants to spend money on a 'cost center' that won't improve profits.
Then these people didn't change their ways when they were given adequate space.
Then these people didn't change their ways when they were given adequate space.