from my stand point, there are many advantages to air over co2.
1. its easier and quicker to fill
2. you always fill it to the same point, as most rigs and bottles have pressure gauges on them
3. O rings dont warp so quickly with air. CO2 kills O rings SOOO quickly
4. its a generally cleaner gas. you dont get discharge out of the gun.
5. you dont have problems related to liquid co2 in the marker, as this can cause your gun to run hot, misfire, and other horrible things.
6. and also you dont freeze your hands/ arms to the marker / bottle when firing alot of shots through the marker
the only disadvantage with air, is that air bottles have to be tested by law every 3 years for safety. where as with co2 its 5 years. oh, and the cost.
hope that helps