I'd have to disagree with all of your statements. Not only are they derived from no experience in the field, you are missing keys facts. Ducts even if air tight will get dirty. If they are not getting dirty, it means they are not working. Your returns side of the duct work is always under negative pressure when the system is operational.Therefore it is always drawing air in. Seeing how dust is made up mostly from dead skin humans shed daily. Around 1.6 Pounds of skin a year per human. Living in your home anything that is air born will be drawn back towards the cold air returns. That is what they are designed for, to draw stale air back to the system to be filtered and sent back through the house.Homes with carpet will actually have cleaner duct work than home with all tile or hardwood. This is because carpets hold onto dust particles and you vacuum them up. Whereas on hardwood or tile, dust can run freely back to the returns. Even if a duct system was sealed 100%, which is next to impossible to achieve 100% would still need to be cleaned. The cleaning may only have to be every 7-10 years for example. Most homes require every 5-7 years. And that's if it was done properly the first time. If you constantly hire people that have no or minimal knowledge of duct cleaning, You home would require cleaning more often. Every 2 years. That's a trade secret, The longer they recommend before the next cleaning, the odds are they did it right. That's not to say there aren't people that lie, because there are. Just have your duct cleaner take pictures before and after cleaning. Not just an outside view. Get in there, picture entire joist liners, Whole main runs and even long round pipes. Also it helps to have a company that guarantees their work. If it's not done right, they will come back and do it until it is. Most companies that do this make sure it is right the first time. Your not getting paid to do the job twice.
The Environmental Protection Agency says that “duct cleaning services typically – but not always – range in cost from $450 to $1000 per heating and cooling system, depending on the services offered, the size of the system to be cleaned, system accessibility, climactic region, and level of contamination” and type of duct material. Consumers should beware of air duct cleaning companies that making sweeping claims about the health benefits of duct cleaning – such claims are unsubstantiated. Consumers should also beware of “blow-and-go” air duct cleaning companies. These companies often charge a nominal fee and do a poor job of cleaning the heating and cooling system. These companies may also persuade the consumer into unneeded services with and/or without their permission. (If you have knowledge of a practicing “blow-and-go” air duct cleaner, contact your local Better Business Bureau to report the company, and your local, federal, and state elected officials to demand legislation.)
Equipment used to clean ducts varies widely. The cheaper the equipment used the more you are just wasting money. This is why it's typically not worth doing. In some situations it may be beneficial, but only after determining and investigating the duct system. This should be done by a licensed HVAC contractor and not a duct cleaner that typically do not hold and HVAC license. (Duct cleaners aren't required to be licensed in HVAC in many areas.)
I think that the coding and permit industry is a joke!! They will not allow the homeowner to apply for the permit it has to be the licenced contractor. It would make since to allow the homeowner to apply for this. But it is all about money!! They really don't care if its up to code because if they did then they wouldnt care who applied and payeed for it, but they want the contractor to do it so if they do something wrong they can fine them instead of just pointing it out and allowing them to do it right.

Well, I had the cleaning done. The handler is definitely much quieter. It was explained to me that with the coils clean, the condensation will be able to sweat off instead of sticking to the dust and dirt, therefore the humidity (lots of humidity in florida) will be removed better. I feel as if it was worth it, but, I will definitely change that filter every 3 weeks or so from now on. I don’t want to pay that cost again.
fact is most homes have bad filter racks. when you insert the filter it doesn’t seal well to the sides so some of the air and dust bypass it and end up in the supply ductwork and into the house. Most of the dust in the supply ductwork settles down and doesn’t get blown in the house but some of it does when the blower starts up or when you put a new filter. If you have allergies or other health problems put in an electronic air cleaner, get your ducts cleaned and run the fan as much as you can afford. you can also get an ECM motor installed which is going to be worth it if you want to run the fan 24/7. If you have uninsulated metal ducts take down the ones you can and wash them. Then put them back and seal the joints. Cleaning the ductwork also improves airflow which is important to AC and Heat pump systems for efficiency. An electronic air cleaner will also improve airflow and static pressure because they usually have a much larger filter surface area. An electronic air cleaner also keeps the ac coil clean like new which means better efficiency and no cleaning needed. AC furnaces sometimes grow mold around the coil because of the condensation. Electronic air cleaners should be able to filter mold spores but the best remedy is installing a UV light next to the coil which will kill the mold.

All of the products discussed above are registered solely for the purpose of sanitizing the smooth surfaces of unlined (bare) sheet metal ducts. No products are currently registered as biocides for use on fiber glass duct board or fiber glass lined ducts, so it is important to determine if sections of your system contain these materials before permitting the application of any biocide.
Some research suggests that cleaning heating and cooling system components (e.g., cooling coils, fans and heat exchangers) may improve the efficiency of your system, resulting in a longer operating life, as well as some energy and maintenance cost savings. However, little evidence exists that cleaning only the ducts will improve the efficiency of the system.