Airstones will help create a balance in the water with the amount of gases in the air above. So it will keep an equilibrium but really won't "help" add much in the way of CO2. The only real way to add CO2 is by using a DIY CO2 canister or one of those hagen systems. IMO the airstone won't hurt but it won't really help either. The plants are alaready creating oxygen in the water during the day and CO2 at night.
If you are supplementing CO2, airstones definitely will hurt, as they will push the water column to air equilibrium on CO2, which is <5ppm, so you will be be wasting your time, money, and effort in supplementing CO2. One or the other, not both.