If you still check the quarantine folder once a week, do you even need the challenge email? At most it will take a few days longer to reply to a valid email and then you can whitelist the sender.
Think of it as "if they respond to the challenge email it'll get in front of my eyes quicker".
Also, keep in mind when I wrote it, I was getting 20-50 spam a day (not counting unsubscribe emails). And part of the motivation was that legitimate emails were simply getting lost (I wouldn't notice them in the sea of noise). Checking once a week would mean scanning hundreds of emails for the few legitimate ones. The challenge-response is more reliable.
(Today I get almost no spam - someone fixed a broken pipe on the Internet).
Also, keep in mind when I wrote it, I was getting 20-50 spam a day (not counting unsubscribe emails). And part of the motivation was that legitimate emails were simply getting lost (I wouldn't notice them in the sea of noise). Checking once a week would mean scanning hundreds of emails for the few legitimate ones. The challenge-response is more reliable.
(Today I get almost no spam - someone fixed a broken pipe on the Internet).