We had it done once, def not a scam! The crap they pulled out of our ductwork was amazing and horrifying! Think about 50 years of dust, toys, small items of clothing and general crud built up, and the air that is supposedly filtered at the furnace then blowing over that before moving to the rooms in the house. Yuck. Our allergies improved after having this done.
That said, I wouldn't do it often, but sometimes, yes!