> - spammers will use your reply to verify there's a human at the other side

For real spam, sure, but for semi-legitimate spam like real businesses and political fundraising, I'm not sure this is actually true. I have found replying with STOP did reduce the volume of political spam I was getting. I think it makes intuitive sense that they should try to respect opt-out signals: you don't want to piss off the people you're trying to appeal to. It hasn't entirely eliminated them, but it seems to have been more effective than Junking them.

Could just be coincidence, of course. Who knows.