Good question. Cored hull sides are definitely the norm among (current) name brand manufacturers.
In my limited experience, cored hull sides are obviously not common among boats that relied on fiberglass as the only material used to construct the hull. Also, boats that were 'splashed' from their original manufacturer and built like a potato chip may lack cored hull sides or any core whatsoever.
You may want to post this in the Fiberglass forum.