Just a correction here: Plants do metabolize oxygen, using pretty much the same process we do, and even produce carbon dioxide as a biproduct of this. This is their respiration cycle, which is the same cycle we use. The only difference is that plants through photosynthesis convert carbon dioxide into oxygen (and sugars), and they convert more CO2 to O2 than they metabolize O2 to CO2. So, as a whole, it's true that plants consume CO2 as a resource and release O2 as a biproduct, but it's not true that O2 is unused by them.

A better example does exist on earth, however--some anaerobic life (eg, bacteria) find O2 poisonous (such life, obviously, is not found sitting on the surface).

So I agree with your general point... just not your example.