Of course, every air bubble adds to 'surface area' and so increases diffusion (of O2 and CO2) - the inside of the bubble where air meets water is a 'surface'. The "you can never have enough air in your tank" (I'm assuming he mean't Oxygen) statement is very true in that freshwater will only hold a certain amount of O2 - between 4ppm and 14ppm if I remember correctly - once it hits 14ppm it doesn't matter how much 'agitation' you have, it won't get any higher.
Kordon Breather bags work because they allow diffusion to take place. The bag material is semi-permeable to the extent that it will allow CO2 molecules to pass through (and as they are bigger than O2 molecules, by defintiion it will also allow O2 to pass through) but will not allow H2O molecules to pass. Just like a large mesh net will let fry escape through it but will catch the parents. The CO2 is allowed to diffuse out of the water as if there wasn't a bag there. O2 is not actually drawn back into the bag, it simply passes through the bag and enters the water by diffusion. The 'Breather bag' is really just a very very fine net!
Sorry, off topic, but it is diffusion at work!
