Why would ANY poster attach additional posts on UNrelated people/subjects into a single thread."
Because they ARE related. By geography and surname. All Smith burials listed on a site, is the generic classification.
"Also, suppose something is in error and needs to be deleted (I've had the Teafor2 posters ask on occasion to delete a post so that they can make a corrected post) and in the middle of a thread this wouldn't be possible."
A deletion in the middle of such a thread would not be necessary. All he/she has to do is edit that individual post to nothing but a placeholder.
Always the defender of the mighty Acom/RW status quo aren't you? It is just so wonderful that they got everything exactly right when they set the boards up.