For the most part, the Grimm Pyrrha theory came about because, if Salem or Cinder had the power to do something like that, it would be absolutely in character for them to do it. Cinder's all about turning one's power against them, and Salem wants to do everything she can to make humanity lose all hope before she kills them wholesale. Wouldn't corrupting someone being groomed to be humanity's next young champion and turning them upon Remnant make complete sense, given those tactical mindsets? Also, the theory works even if she's dead. One would think that, because the Grimm are soulless beings, that the corruption would be actively resisted by one's Aura/soul. Thus, it makes sense to corrupt the recently dead.



That, and the potential mass character development would be IMMENSE. You have a "Fighting Your Friend" dilemma with Ruby and JNPR, you have the whole of Remnant getting their worst fears confirmed about Pyrrha based on what they saw her do to Penny (Because of the CCT going down, not only was Pyrrha killing Penny the last time anyone watching the Vytal broadcast saw her, but those people don't know she's dead), and if certain things have been interpreted right, the possibility of a resurrection exists in a way that doesn't cheapen her death, and can't be abused, thus cheapening death as a concept.