I'm thinking that firstly it's pretty rare for some other strategy to be better when Rebuild is on the table.
Beyond that, if one person starts going for Castles, other people need to as well. Yes, you can only name one card when playing Rebuild, but you do know what's still in your deck, if you're careful.
You could Rebuild stuff into Castles, Castles into other stuff, and I'm not sure Castles into Castles would be especially infrequent either, if multiple people were buying them.
Hey, in a few weeks' time, I can try it and see what happens. (-8