Spoiler: More on the tangent
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Actually...
Discounting the murder of the Soul King, the only time in canon that we actually see the Balance of the worlds in danger is after the Wadenreich systematically slaughtered both the population of Las Noches and sent hunting parties to scour the deserts and forest of Menos of every Hollow they could find, killing thousands of menos in theprocess.
Thousands of individuals composed of hundreds or thousands of souls each--millions of souls at a minimum--being destroyed in just a few days was what it took to noticibly upset the balance.
Which was then immediately fixed by summarily executing just a few thousand rukon citizens, sending just a few thousand souls down to earth.
This strongly indicates that mortal souls on earth are far more important for maintaining the balance than spirits in the spirit world by a significant margin.
By sealing away spiritual power or killing mortals with spirit powers and bringing them to soul society, they may, in fact, be slightly disruptting the balance.