You are, of course, absolutely right re. spam and rightfully so. On the other hand, I regularly get spam reports re. 100% legitimate mails, often in the middle of longer email conversations and so on, and where one side of the conversation marks entire email threads as spam.
We always follow up on spam complaints and the one thing we do get often in cases like that is that the other party only wanted to delete the emails and didn't realise there were other consequences.
Yes, there's a lot of people that knows how to use the "Mark as spam button". I still, however, maintain the position that a lot of other people don't know how to use such features.
I would unsubscribe and delete, but have not really thought about marking as spam or not yet. Just interested in some input there.
If I would get it from a company where I have not signed up the answer is of course obvious.
When I get the old "you'll be removed from our list within 10 days" message, I generally reply to it with a quick message: "Please remove me immediately or your next message will be marked as spam". Many times the sender has complied with a personal response provided someone saw my message, even for some large companies with "no-reply" emails.
For smaller companies - especially the mom and pop type operations - I'll usually give them a friendlier notice. "Hi, I unsubscribed and I'm still getting emails. Please remove me as I'd really rather not mark your messages as spam". I usually get a friendly personal response with apologies and a removal.
In all cases, after the company has been warned, I don't think twice about sending them to spam.