Air stones don't do a good job of mixing oxygen into the water. The tiny bubbles don't agitate the surface much. If you remove the air stone and place the end of the plastic tube at the bottom of the tank, then you'll get the surface movement needed to mix more oxygen into the tank water.
Remember that a high oxygen environment is good for the fish, but not for the plants. Oxygen is a plant byproduct. That means the plants work to remove it from the water.
I may be mis-interpreting what you are saying, but here goes anyways:
1) The bubbles coming out of the airstone don't exchange much O2, but they do a good job of circulating water in the tank. That circulation of the O2 rich water at the surface back down, and the comparatively depleted water at the bottom to the top for gas exchange. In this manner they do a good job of getting O2 in to the water.
2) A large number of airstone sized small bubbles does a better job of moving water than the same amount of air running out of the end of a tube. If your tube with no airstone is moving more water than the same tube with an airstone you probably have a blown diaphragm or check valve in your air pump. I've yet to see someone put an airstone anywhere other than near the bottom of the tank.
3) Oxygen is a plant byproduct, but all of the other aerobic processes in the tank (fish, beneficial bacteria) really love it. People go to a lot of trouble with wet/dry filters to get their O2 up. I've seen two cases where dissolved oxygen has been pushed above equilibrium in an aquarium. In both cases it was plants (with CO2 injection) doing it, and the plants were quite happy in both cases. If you have any documentation where high O2 hinders aquarium plant growth I would love to see it.
4) Incidentally unless you have a lot of decomposition in the tank and very little surface exchange (this happens in some lakes), your plants are also keeping your CO2 below equilibrium and the extra circulation will (ever so slightly) increase that .