anyone who's taken an engine apart knows that there are more coolant ports and flow paths than the HG allows us to use. question is why..? why build the block and head to have that many ports then not use all of them..?
I believe its something to do with keep the engine up to operating temp. Remember that Toyota probably never imagined people would be pushing 200% above stock power on these engines so felt no need to have the waterways open. However it was a good design to allow them to be open with just a gasket swap.
they are blocked off to aid engine warm up speed, they also aid in water flow direction. downside to this is that water between the cylinders does get slightly hotter than other areas.
the earlier 2e engine (the 4e is heavily based on this engine) has these water ways open slightly, and the head gaskets are interchangeable. the athena gaskets for the 4e are actually mls 2e gaskets