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Ernie never comments on the specifics of his design choice...perhaps you could email him and ask.
As far as winglets go, they reduce drag both by reducing wing tip vortices and increasing the effective aspect ratio of the wing which increases lift and tends to further reduce drag. A number of airlines have retrofitted winglets to Boeing 737-300s, among others, to save fuel.
Obviously, the height of the end plate on the VW is partly determined by the placement of the wing element and also by the extent to which one wants to remove the inevitable vortices at the tip of the end plate from the wing element. The end plate also functions as a fence to some degree, influencing the airflow from other areas of the body depending upon it's size and placement. (There is an old joke about the fences on the wings of Soviet Migs that the fences were "to keep the air from defecting to the wing tip.")
I believe that Ernie commented somewhere that the increase lateral surface of the wing,s end plate improved the Bug's (lateral) stability in cross winds.
Of course there are practical considerations in the mounting of wings. There are wings mounted on the trunk lids of cars which have the support/end plate angle out so that the wing can be wider than the trunk lid and yet maintain the ability to open it.
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