Comment by jacktoole1

Maybe more importantly, they would know whether their message was classified as spam (or a pretty good heuristic thereof). A naive bayes classifier for spam relies on the spammer not knowing whether the message was marked as spam or not. Such a classifier isn't difficult to fool if you can test how a given message is classified. (Presumably gmail's spam filter is more advanced these days, but the idea that it is easier to fool if you can tell the result still seems reasonable.)