I'm not expert (so let me start out by saying that.) But I'd suspect that you might 'experience' that effect but I kinda doubt it. Here's my thinking ...
Well, first off, the folks that do this for a living would be a bit biased. It is their living, eh? (One wouldn't be so willing to bash their own livelyhood.) Yes, ducts do collect dust especially if they're used on occassion. WHich would, I think, eventually blow out over time as it becomes dislodged from inside the duct. At the same time, if one doesn't clean/replace the system's filters, dirt and dust may find its way around the filter and into the duct and then back into the main house.
At the same time, you have to know what the sources are for dust. Clothes lint, carpet lint, furniture lint, dirt particles, just to name a few. They're ALWAYS in the air and will be blown around, to boot, by the heating/cooling system.
Now, as far as being a waste of $$$, it may be worth at least a 1-shot cleaning if they haven't been cleaned in 30 years. I don't know - It's a tough call. Over the years, I've had to open up and peer into my ducts, and I was surprised at the things I found in them. I found stiff from when the home was constructed (like small 'blocks' and scraps of wood and nails), kid toys, pocket change, a rag, pens and pencils, and so on. All these things will 'collect dust', and over time, release it into the system to possibly be blown back into the main house. So if you want to take a peek into a few return ducts and heating/cooling vents, that might be a good start in determining the 'status' of your ducts. Of course, you'll need to remove the covers and possibly use a flashlight and mirror to help. Go ahead and 'run' your hand on the inside BUT DO BE CAREFUL OF SHARP EDGES. Other than that, wait to hear what others have to offer.
My best to ya and hope this helps.
Jay J -Moderator
PS: Again, if they're cleaned, you may notice a slight drop but I think you can forget about a COMPLETE drop in the level of dust in the house ...