Lot of people do it after remodels when contractors don't do good job covering. I wouldn't say it's a scam but what I don't like is how do you know they actually did the complete duct etc. You might see if they have cameras they can show you your ducts first to see if you think it's worth it and pay for their time doing that. Changing filters is something that if you set a reminder on your phone is a great way to maintain your system. Me personally unless I had a remodel done I'd just keep up on filter changes.