There is no real "one size fits all" answer to this. Airstones increase surface agitation, there is very little Oxygen or CO2 disolving into the water from the stream of bubbles itself. In highly stocked aquaria, surface agitation is good as that is where Oxygen is disolving into the water. In most cases however, the outlet from the filter is agitating the surface, the role of the airstone then becomes simply decorative, some people like them. Personally, I do not.
Surface agitation allows CO2 to come out of solution, so if you have a CO2 injector, the airstones are driving the CO2 out before it is getting used, you are thus pretty much wasting your time injecting it.
With low to medium light plants, CO2 injection is not necessary, their metabolic rate is slow enough to use the available CO2 from your fish etc. High light demanding, fast growing plants will benefit from CO2 injection as there will not normally be enough - without supplement, lack of CO2 will be the limiting factor for your plant growth.
You can buy all kinds of CO2 systems, there are pills and potions which you can buy, which are clean and easy but don't work very well. Simple CO2 reactors using yeast and sugar can be bought, or very simply made, these work fine, but require regular maintenance, (I use home made reactors). There are also very fancy systems that use high pressure CO2 gas bottles - these work fine, but are very expensive.
I hope that covers the questions! If not, ask away!