Could probably take out the intake fans and put a mesh screen in place of those. What this will do is filter dust very well, since you'll be sucking all the air through the case via the rear exhaust fans. If you look at high end ATX cases, you'll notice how there's usually more overall CFM as exhaust (Rear, top, psu if on top, and somestimes side is used as exhaust), since you don't really need intake fans with proper case filtering. Usually the intake fan is there for cooling HDD's anyway.
The exhaust fans in the back will create the suction needed to pull air thorough your design regardless of intake fans. I'd suggest trying to replace the front with some type of dust filter and see what that does. I'll bet the temps will stay the same.
It's called redundancy my good friend
We can safely assume anyone who build a box like this wants to have it run 24/7 at which point we really don't want to risk having one of the fans die and thus increase our temps, this would especially be critical if the box contains 4x6990 or other dual gpu cards as they produce an enormus amout of heat...
I see your point about dust though but it's not a big issue for me as my rig is placed on the roof where no one ever goes so the dust isn't really in circulation.
And yes Chick, it's a bit expensive but what does 80$ worth of fans matter if you've just bought 4x5850 @ 130$ each?