While I liked the atmosphere of the Old Chateau, I found it to be an otherwise lackluster area in terms of the story. Much like the Abandoned Ship in R/S/E, it served only as a place to gain a couple of rare items, and in this case, Rotom as well. I really think the backstory could have been elaborated on and turned into a quest either in the main story or the postgame. Team Galactic could have been after Rotom to use as a power source for one of their machines or something (at least in Platinum where it can be caught before obtaining the National Dex). According the Bulbapedia, the note found in one of the rooms was written by Charon, indicating he might have had some interest in the place already, but I'm not sure about him growing up there.