Even if you don't reply, typically the spammer will receive an 'undeliverable notice' from invalid accounts, so silence ought to indicate where an addresses exist.
right, and what better way to ping for active accounts than with a (in this case, pretty crappy and almost worthless) bayesian poisoning attack?
In addition to that, maybe some spam filters work by automatically putting a sender on a whitelist if it gets through once, and then after the sender is on the whitelist, he can spam with something real later.
silence indicates the address exists. a person clicking the unsubscribe link indicates that someone is actually reading the email sent to that address.