Yep, Rutra, you're sure correct about 'no noise' in rays. Good catch there. I still would rather use GA  instead of GI for his scene. GI, unless the quality setting is jacked up pretty high, will create 'splotchiness' or noise in some areas-- areas where there is already quite a bit of noise to begin with-- like the vegetation. So, from a quality standpoint, I'd rather spend the cycles on better aliasing than computing GI.

Also, many times GI can create a large distant object appear to be a small object in a small scene, not unlike how DOF affects scenes similarly. Think of a large space station, or very large building set off in the distance. GI may in fact show affects of reflected light and shading even though none would be visible at that distance. This confuses the viewer into a scale misunderstanding of the scene. Typically, if all scales as applied are correct, this type of artifact may not be noticeable. But, as Vue artists, we generally tend to 'cheat' on our scales, and make building smaller and mountains smaller so to 'fit' in our world scene space.

A nice compromise for this may be AO as you can set the affected distance with it.

I'm not sure I understand your point about sun changing to a shadow map when a volumetric model is used for the sun. In my version of Vue, it maintains itself as a ray-traced light and shadow when set to 5. You might want to check it out for yourself.