As others have correctly pointed out, surface disturbance drives CO2 out of the water before plants can fully use it; it also brings more oxygen into the water, which can also be detrimental to plants. [I explain this more in my article "A Basic Approach to the Natural Planted Aquarium" at the head of the Aquarium Plants section.] Assimilation of CO2 from the water through the leaves of aquatic plants takes longer than it does in air, about 4 times longer, so driving the CO2 out is only removing valuable nutrients for the plants.
As for airstones, they should not be used night or day in planted tanks. If you add CO2 via diffusion during the day, an airstone at night can help to dissipate excess CO2, but that is another matter entirely.