Oxygen diffuses into water much easier than CO2, so being that you are only injecting ambient air into the tank, the majority of the surface area in contact with the air is being used to absorb o2 with vary lilttle contact with CO2. Also, you may actually be causing more CO2 to offgas the tank by causing turbulance at the surface.
Anyway regardless of the true results, I personally would not like to see all the bubbles floating throughout my tank. It looks ugly to me.
Actually, that is a bit backward. CO2 dissolves into water much easier than O2 does, and it also comes out of solution equally easy. So, this method does speed up the absorption of atmospheric CO2 into the water, but it also speeds up the loss of CO2 to the air, due to the water surface disturbance from the bubbles. So, you will get no net gain in CO2, above what you get with a quiet water surface, but you are probably getting more of the CO2 down into the lower water to replace that used by the plants. Just good water circulation in the tank will do that too. If you don't like seeing those bubbles, just using the Koralia to keep the tank water moving will do the same job.