[Formendacil]

The direct reason for Frodo's PTSD would, I agree, be simply cause and effect. If it is necessary to the plot to have someone stabbed, you logically have that someone feeling the effects of a blade in their body, blood leaking out, and the physical issues that follow. Similarly, if you put someone through what Frodo went through... you get someone turn out much like Frodo.

Which does not mean it cannot have been therapeutic for Tolkien to write it, in some form. To know what something is like, it generally helps to have either experienced or observed it. Whether Tolkien experienced PTSD, I have seen no evidence yet, but he would have had to have observed it firsthand. And if it had affected him, either personally or through those around him, which is likely, it's the sort of thing that would easily need some exorcising.

The importance of speaking of a trauma in order to recover from it is a generally acknowledged fact, I would say. How much actual talking about PTSD would have gone on in Tolkien's generation? Probably little enough, I'm guessing, that he would still feel, perhaps subconsciously, a relief in recounting it vicariously through Frodo.