Ducts that are so dirty can be quite difficult to efficiently clean, and the best option may be to replace them. With proper maintenance, no, there is no need to clean the air ducts. Instead, your local HVAC company should seal or replace the ductwork. If you have dirty ducts, you have a problem somewhere that needs to be found.
It seems like a logical maintenance activity, but cleaning the air ducts in your home's HVAC system may not be as good an idea as it seems intuitively. Your HVAC unit is designed to last 12 to 17 years if properly maintained, so if you're nearing the end of the unit's lifespan, it's time to replace it.