While they don't put out anywhere near the 42 cfm claimed on the package, they still put out a good stream of air. My case sits on the floor under the desk, about a foot from the keyboard and I can feel the air moving on my fingers. If you look in the air cooling forum you'll see a post I did a while back where I did a Silverado on steroids with 2 of these blowers, and while the results weren't overwhelming they did come close to a delta screamer.
Because it's under the desk and there's only about an inch between the top of the case and the underside of the desk a blowhole wouldn't work as well, and would be noisy. If you were relying on the blowhole as one of your primary exhausts, then yes one of these wouldn't be enough. But most of my heat is going out the psu and fan by the cpu, this is just mopping up the leftovers.
Plus, you don't lose the top of the case for piling up crap
