If sender reputation is tracked that way, maybe reporter reputation should be tracked the same way. If someone reports a legitimate confirmation mail that they signed up for, as spam, then it's the spam report that shouldn't be taken seriously.

Overeagerly classifying legitimate email that someone subscribed to as spam is no better than classifying spam as legitimate. Especially on a system like GMail where one person's incorrect classification would lead to other people not receiving their subscriptions in their normal mailbox.