Airstones rarely have a place in a sw tank, but they can make a big difference in raising pH by off gasing CO2.
Both Ryan and Oxymoron are right. The rippling of the water surface does break the layer of greasy organics on the surface, however if you have a sump with an overflow the surface grease should not be an issue. An airstone
does increase O2 in the tank (the efficiency of this varies with the quality of the airstone and how fine the bubbles are), but if you are running a skimmer (that fits the size and bioload of the tank well) a skimmer is essentially a giant airstone when it comes to O2 and CO2 gas exchange. If you are using cheapie $2 airstones you aren't going to produce much O2 exchange. Buy some super fine wood or glass airstones, and the results will be much greater.