What is the purpose of direct illumination vs global illumination? Both images look natural to me, although it makes more sense that global illumination is more applicable in real life because objects will bounce off lights. However, direct illumination looks just as or even more natural to me.
One could argue graphics is a matter of taste - if as the designer you believed direct illumination was enough, then of course you could stop at this point. Personally, I kinda get a video-game vibe off this image - the extra dark spots might be from not being fully rendered in a game, and only become more detailed when you get closer to them. The dark spots in this picture in general give it a lot of contrast, while in real life I'd expect more muted colors (almost "hazy" from the light) on a bright day.
I mean, also global illumination is a much better approximation to what reality is, which I think generally is a good goal for graphics. The "video game" vibe is a weird artifact of inadequate computational resources, sort of like how 8-bit music reminds us of game boys or whatever. But that's beside the point. The reason why direct illumination might look "just as or even more natural" is that it could be real for a given scene, but it's not faithful to this scene.
Another answer to Katleej: It can also be an artistic choice. You often see in horror games very constrasty direct lighting that makes certain parts of the game completely dark. This gives up reallism, but makes the game scarier since you don't know what's in the dark even though there's a lamp 10 ft away.
Maybe I misunderstand direct illumination -- I thought this image would only show the sky since the sky is the source of light (much like how the first image of spheres only shows the source of light). The walls/columns are not light, and light from them is a result of one or more bounces of illumination.
@ariel, if you refer to slide 1 of this lecture, the one with the balls in the Cornell box, it seems as if direct lighting is equivalent to the one bounce lighting we implement in the project. So each surface in the scene only receives light directly from a light source (in this scene, the sky)