Not that I frown at people deciding to delete their posts for the sole purpose of de-ranking themselves and on the other hand it doesn't in anyway fault forum rule in doing that.
But am just raising a concern I conceive in mind after reading your comment, like in a situation where an OP gets to delete a particular post of his which contains quality information, solves certain regularly asked questions that might be of a burden to a newbie or any member rank at all. So to me I think if many people continue doing that we might lose some answers we could have easily sight or pointed to in the forum when we meet similar questions that such reply/answers could have resolved or been of help to but then it already deleted by the OP.
I don't know if you understand the perspective am talking from.
I understand your view, but, since these people have control over their contents they can do to it whatever they wish. Although, their answers can't be the only one in a thread, tens to hundreds of responses are seen in a thread. What could be of importance is deleted threads, which goes away with other people's responses under the thread. Thereby, reducing the post count of the owner and that of the members who responded to the thread. I've seen some users who complained about miscalculated post counts not knowing that a thread they responded to in the past must have been deleted, either by the Op or the moderator.