Obviously YMMV, but......
Using reduntant fans soley to reduce the amount of dust inside your case seems like a waste of noise. If you want to run positive pressure, just use an intake. Using a balanced intake and exhaust is acheiving nothing but doubling your fan noise: The airflow through the case is the same, the cooling is the same, and the air pressure is higher with only intakes than with both intakes and exhausts. (If your logic is that "higher air pressure= less dust" then wouldn't you want the air pressure as high as possible?)
I would rather trade a little dust accumulation for noise. Trading 5 seconds of blowing the case out with canned air once every 6 months, for reducing the fan noise 24/7 seems like a more than fair deal.
As for dust damaging components:
Nothing that dust does cannot be undone by a bi-annual blast of air and a CD lens cleaner. Yes, dust can clog the fins of heatsinks, but even after months of use the temps will only raise a couple of degrees. If that couple of degrees if enough to push you system over the edge, you've got more serious problems than dust.
And yes, dust can get inside optical drives. But only a tiny amount of dust will get into a closed drive. Dust will follow airflow; if 99% of the air in coming in the intake, then 99% of the dust will to. I'd willing to bet that the vast majority of drives that failed "due to dust" were actually simple mechanical wear-outs, and not dust related at all. Blow a little canned air into it once and a while, and run a lens cleaner through it.
Perhaps my lack of understanding of this dust obsession is because none of my machines ever get that dusty. I was inside of a machine of mine yesterday that's been running continuously for nearly a year without being opened. Inside there was some dust; a few dust bunnies in the HS fins, and a bit on the trailing edges of the fan blades, but that was about it. The was only a thin film of dust on the bottom on the case, hardly enough to even write your initials in. And this case was running "negative" pressure, unfiltered, while sitting on the floor in a high-ish traffic area.
And trust me, I've never been one to worry much about keeping the house spotless either.
Now I have seen some machines that were so dust filled that it was impossible to even tell what sort of hardware was in them. My personal fav was a woman who spent years sitting next to her machine chain-smoking and petting one of her 7 cats. That entire machine was covered in cat hair that had been "tarred" to it by the cigarette smoke.