The article linked to photo (click on photo) from the Washington Post says studies show dust in ducts usually stays there and does not noticeably affect HVAC efficiency nor dust levels in the house. After duct cleaning dust levels actually increase temporarily in the home, which makes sense. I recommend duct cleaning when moving into an older home, if for nothing else, to start out with your own filth, and, if there has been a major mold event in the home. The article also mentions the need to have clean filters and coils...issues addressed in other posts.
The worst thing I've found in ductwork is piles of racoon feces, which harbors a dangerous bacterium...also a reason I wear a mask when entering an attic or crawlspace.