The dust that accumulates very rarely ever comes in through leaks in the HVAC ducts, but leaks in the supply ducts, including where register boots penetrate into the conditioned space, can cause the inside of the house to be at a lower pressure than outdoors.
This causes air to leak into the house through every available opening, so a lot of the air, even most of it, comes in from or through some very dirty areas.
If you have recessed light fixtures, they are a huge leakage point from the attic, unless they are specifically manufactured and installed to be air tight.
Yes, will look at.
Other sources that should be looked at are old carpet, leaky vacuum cleaner, poor dusting techniques, pets, people, etc..
No carpet, super duper vacuum, dusting weekly and yes a dog.
In some homes, most of the visible dust is dead skin cells from the people living there.
Again, no air filter mounted anywhere in the HVAC system will do much of anything to reduce a visible dust problem.
So what good are they?