Funny enough, this method you're talking of called the CO2 bell method is the least effective.
CO2 bubbled through the correct medium, (a diffuser in the tank or inline) is far more efficient!
Do a quick google of most efficient diffusers!
Sent from my iPhone using Tapatalk
Well, I'm not sure you're using the correct word. I'd say that using the bell method is extremely efficient, assuming you're not losing any CO2 bubbles out the bottom. So in terms of dissolved CO2 vs lost CO2, yes, it's efficient.
However, it's just not very effective. The amount of CO2 that can be dissolved is minimal using this method.
To the OP: using a regular airstone, you're correct, would be incredibly wasteful. The bubbles are just too big and go straight to the surface, losing most CO2 to the air. That's why diffusers exist - they've got much smaller pores, creating smaller, far less buoyant bubbles which get carried through the tank by current created by filters and/or powerheads. The longer the bubbles are in the tank, the more CO2 is dissolved. And the smaller the bubble, the more total surface area given the same total amount of CO2 than with bigger bubbles (you can do some geometric math to confirm this).
Basically, we use far too much CO2 to use a bell diffuser with. A reactor, however, can give you 100% dissolution, so zero CO2 goes to waste before it's dissolved.