You mostly use it for that first reason, to hax Thomas some quick cash.
I'm not aware of any special scenes that come from decorative items that you put around the castle. You DO get special scenes if you take certain bath items into the bath with you, but it's really not worth it. The only good bath stuff comes from taking specific character combinations in there to have private discussions.
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Thank you, Eddie Guerrero.
http://img.imgcake.com/Icon/Punkjpguh.jpg