I feel OK using it with companies I recognize. Often, it's because I signed up for the mailing list accidentally, or to receive a discount on something. They're not out to scam me, and they'll generally honor the unsubscribe request.

I'd never use it for some scammy spammer.

GMail does a pretty good job of separating the former from the latter. If it ends up in my inbox, and I don't want to see it any more, I'll unsubscribe. If it ended up in my spam folder, it's almost certainly a scam.