This company didn't create this bacterium, but it did isolate it, bottle it and sell it.

The reason I would give for not replacing N 100% is because of energy requirements to reach top yields.  This symbiotic relationship they mention requires something from the plant - energy.  Energy that could be diverted to kernal fill.  Energy is the ultimite yeild barrier based on solar radiation driving photosynthisis carbon dioxide and water.

It takes alot of energy in the form of natural gas to change the atmospheric nitrogen to a higher energy state.  Nearly all fertilizers contain not only the nutrients, but also energy in whatever form they come in and they replace some of the energy requirements in the plant /soil system.

The one thing about using this product I would guarantee is that the more N you do put on your field, the less energy your plants will trade away to these bacteria for N and less these bacteria will provide N.  Similar to the way Microrhizza(sp) quits working at providing P and starve when you heavily fertilize for P.  The plant just has no use for what they are providing. 

You have to keep your plant a little hungry for N if it is going to trade away energy for it, but that traded energy cost a small amount of yield.

JMHO

P.S. to add, the key to making something like this work is to find other ways to save the plant as much energy and make it as energy efficient as possible

P.S.S  It is interesting that this bacteria will live inside each cell, rather than live in the soil and colonize on a root.