Personally, I view "mark as spam" as a way for me to cast a vote on bad email behavior. No, I do not want to receive the latest news about your product that I bought 5 years ago -- yes, it is spam when you email me about it. It is spam when you email me asking for money just because I attended your group's meetings a few times in college. It is spam when you send me an email that does not render unless I enable HTML. It is spam when you drop my friends' names in an attempt to get me to sign up for your website.

Spam is not just about scams, malware, and crime. It is about the annoying deluge, the fact that I have to open a browser and enable Javascript from 20 different sources when I unsubscribe, and generally about unsolicited, unwanted email trying to get me to do things I am not going to do. Filters, including a spam filter, are what keep my inbox under control.

Or you can just mark as spam. Most of those "newsletters" and advertisements follow a pattern that is easy for Bayesian filters to recognize (I speak from experience). Your spam filter has been trained enough when it preemptively removes such mail from your inbox.