It can be worthless and temporarily harmfull to you as it was to me. My
ducts are 50 yrs old with floor returns. A very expenive 10000$ I was
told large Hepa vac was used, the size of a 5 ft refrigerator and
sanitiser was used. It cleaned them but then I got sick for a week, I
guess all the sealed in mold , cat dander, [ im alergic to cat ] , old
tennants had, etc etc was loosened into the airstream even though the
job was thorough.
Is it necessary, well it depends if filters were used properly and dust
is hard, no. If the house is empty and there is mold maybe. Get a few
pros to look.