Well I'm talking about having both an intake, and an exhaust being useless. I think you should have either/or, but not both.
Now having both will help, if....
1. You need it to blow on something specific, like a HDD.
2. Your case has lots of holes in bad places. Like a big hole in the side up at the top that would let air in, bypassing the rest of the case. (Duct tape would fix #2)
Other than that, nope. Well, at least not enough to be worth doubling the amount of fan noise.
Now just to cut any detractors off at the pass let me add:
In theory....only having exhaust fans results in lower air pressure inside the case, which reduces the heat transfer from the heatsinks to the air. And the lower air pressure results in more dust buildup inside the case.
But....At the airflow rates we're talking about the air pressure drop is tiny. (In fact if anyone can even find a measurable difference in air pressure quoted reliably anywhere, I'll send you a cookie) And the change in thermal conductivity due to this tiny change in pressure is even tinier yet. (In fact I'd bet the increase in case temp due to the extra intake would offset the drop) Axial fans simply cannot produce the kind of a pressure gradiant that would be required for these effects to become important.
And dust is not a good enough reason, for me at least, to double the amount of noise being produced by my case fans. I think most of the dust reduction from intake fans occurs by the act of the fan blades themselves actually catching the dust. Use a filter, or just buy a can of compressed air and give the inside a shot every month or two.
OK, that's enough CYA for one post.