Peter,
Welcome to CSC! I think the only reason Pascoe didn't discuss plywood as a core is that it's higher weight basically puts it in a whole different category compared to either balsa or foam. It clearly has plenty of strength and it's cheaper, so it's the first choice whenever minimal cost is the top priority. Nothing really wrong with that, and Moesly used it in the deck of the
original 21 and the 19' Bowrider. He switched to balsa core when the inner liner was introduced with the 20' hull, probably to save weight, as he was very much aware of the impact of weight on performance due to his aircraft and racing background. Potter continued using balsa core decks in all of his boats, but I believe the 1980's and later models by SeaCraft Industries and Tracker may have gone back to plywood decks to cut costs.
As you probably know, keeping plywood dry is even more critical than for balsa due to the difference in grain orientation. Unlike balsa where the grain is perpendicular to the surface, the grain in plywood is oriented parallel to the surface and rotated 90 degrees in adjacent layers. Any water intrusion will be wick along the grain in all directions, so the entire deck can get soft much faster than if it were balsa core.