I had read that it was because they were exposed to atmospheric oxygen, the bacteria worked more efficiently. I'm not sure really because bacteris work very well completely submerged, so they obviously don't need atmospheric oxygen. Also, only the bacteria right at the surface would get any, unless the idea is that the water is oxygenated as it moves through the filter by a lot of water movement?
I have heard people are happy with them though. A drawback for me is when they are like the ones in aqua one tanks, they sit as a kind of shelf over the back half of the tank. I have a couple of small tanks with similar design and it makes lighting the tank difficult. You couldn't grow real plants at the back, just where you want them!
