I don't know of a way to calculate the effect plants have on water chemistry. It's minimal. In nature, water does most of the work keeping the environment clean. The constant exit of polluted water and the incoming pure water to replace it keeps the environment stable. It's the same with our tanks. Filtration by plants or mechanical means does little to keep the water clear of pollutants. The large, frequent water change does most of the job.
One positive effect of plants is removal of nitrates and ammonia. If your tank is getting phosphates from fish food, or otherwise, plants remove that too. Otherwise I agree.