On the top of my head one of the points of having `glFlush` is that gl buffers are cleared. Especially since in WGL1+ (and OGLES2+) we are explicitly copying into graphic memory, that could e.g. prevent memory from overflowing or some other benefit for larger rendering. You *might* be right about this having no effect when you return from the event handler shortly after, but there may also be the case that you do a second rendering in the same event handler.
Before we do some premature optimization I would rather have some proof to justify not sticking with the spec (which is fine).