Even though these get cleared out, its still extra work for the GC.  Presumably most of the time the display size is identical to the last one, so I wonder if the existing screen size might be passed to the primitive to test and avoid creating a new new point if its the same.  Off the top of my head, something like this (untested)...  

  MorphicUIManager >> checkForNewDisplaySize

Display hasScreenSizeChanged ifFalse: [^ Display].

DisplayScreen startUp.

World restoreMorphicDisplay.