ad 1. I'm not sure if this is the case for gmail but when you use a spam filter that is fed by many people there is a chance that you're classification of that mail as spam affects other people. If you use some sort of self-adapting, learning spam filter, you will be affected too because you mark mail as spam that looks like normal mail.
To put it simple: the spam button is for spam = unwanted mass mails, mostly ads. For unwanted mail from specific users, you should create a filter.
My personal take on this though is: People who found out how to subscribe to a mailing list, should also be able to unsubscribe.
1. It's not "harsh" to mark anything as spam in your email; the only person it affects is you. The sender will not, of course, even know whether you've classified their stuff as spam.
2. Some commenters are claiming that it's not that much extra effort to visit the sender's site and unsubscribe yourself. This is manifestly not always the case, and more to the point, the user does not KNOW in advance whether it will be the case, when starting the process. However, hitting a plonk button in your mail client will always have a known time-suck-factor in advance.
3. Hitting a plonk button is also, of course, guaranteed to be at least one page-load and one click easier than any possible solution involving visiting a sender's website. Even if there's an unsub link in the spammy email itself, a plonk button is still one page-load less, and easier to find from a UI standpoint. So in sum, plonk is guaranteed to win from on ease of use. Not even a debatable question.
4. Some argue that taking time to unsubscribe yourself via the sender's website is just "the right thing to do", given that you subscribed to their emails in the first place. It seems to me that it should be incumbent on the marketer, not the customer, to make unsubscribing as easy as possible, and to "do the right thing". This should be the default.