Air bubbles simply rise to the surface and pop. In saltwater aquaria, we are concerned with dissolved oxygen levels -- oxygen that is dissolved in the water that you can't see (fish get dissolved oxygen from the water to survive, not from taking air bubbles into their gills). Adding an airstone to a tank adds a tiny bit of circulation, but does not add any measurable amount of dissolved oxygen to the water. The oxygen simply rises to the surface, the air bubble pops, and the oxygen is released back into the air. This is why adding air stones does not help to increase dissolved oxygen levels.
In fact, microbubbles of air can be very irritating to fish and are detrimental to their health if they are forced to swim through microbubbles frequently.
As for airstones, in saltwater, they quickly get clogged with salt and stop working anyways. That, and the breaking of bubbles at the surface will cause very irritating salt creep problems on anything that's nearby.