There is a lot of good, general information in the article. As with anything, the homeowner should do their due diligence in getting enough information to make an educated decision. Should duct-work be cleaned? YES! Does it need to be cleaned annually or even every second or third year? Not necessarily. It all depends on a bunch of variables. You're right that a well designed and balanced system will deliver the right amount of air flow, but sometimes conditions outside the home/business, make it a necessity to get the ducts cleaned. As for mold? NO filter is going to address a mold issue. If you have mold, you have a moisture issue that needs to be addressed. Also, in my 38 years of experience in the HVAC and sheet metal industries, washable filters are one of the reasons ducts need to be cleaned. They are no where near efficient in cleaning/filtering the air to the level that they should.
One of the difficulties encountered by many air duct cleaning service companies is that their attention is not really on air duct cleaning. For example, many heating and cooling companies offer duct cleaning as a supplemental service, but of course their primary business is installing and servicing heating and cooling equipment. For these companies, air duct cleaning is not always given the attention it deserves and is often done by less experienced employees whose interest is not in duct cleaning but in repairs and installs. And it is almost always done with inferior air duct cleaning equipment. Still other firms (like carpet cleaners) offer a large variety of services and claim to “do everything.” Because of their lack of focus they may not actually be very good at many, if any, of the services they offer. They cannot simply buy an attachment for their carpet cleaning machine and magically become a great duct cleaner.
After one company just took a cursory glance at our A/C and found nothing wrong, I called HVAC. I called them mostly based on Michael W's post on 8/10/14 where he told us the owner, Alberto, took his feedback and took it SERIOUSLY. That speaks VOLUMES to anyone who has been around long enough to know the characteristics of the serious players are and those who are the wannabes.