Basically, i figure lighting is for 5 (interconnected) things:

1. Defining form and contour (naturally), which greatly affects:
2. realism.
3. Adding clarity and readibility to your composition, to improve your image's:
4. Meaning and story-telling ability.
5. Adding drama.

For outdoors in sun, i usually go for GI and just sun for distant objects & terrain, but use point lights for fill, volumetric spot lights for sun beams and a 2-3 light (photo portrait studio setup) for major characters.

Depending on the scene, i'll also add prop lights, like a point light inside a street lamp at dusk or such. But if the prop doesn't cast light & shadow onto other objects, i do this:
When i need small visible colored lights (e.g. running lights on an aircraft) i don't use light sources. Much less expensive to just use a small glowing primitive cylinder or sphere  

Caricaturing (emphasizing) the different color temperatures of sun, skylight and prop lights appeals to me. For instance, making sunlight very orange, skylight very blue, etc. to add drama. And my protrait lighting rimlight (light from the back of a figure to outline just one side of the figure's edge, halo the hair, etc.) will often be strongly colored.

I avoid flat lighting or zenith lighting and mostly use sunlighting near sunset time, sometimes with a visible sun at the horizon or an implied sun behind a cloud or prop, but with a strong volumetric beam implying the sun's position.

Like everything else in your scene, your lighting ultimately comes down to just what you are trying to express in each image. So, the scene must drive the lighting.

With a life-long love of light and color, and some decades of experience using them, in light shows, light sculpture, inventing projection effects machines and doing fine art photography (and now digital art), i consider myself a lighting expert. You can find some Vue lighting tutorials on my site and a thorough discussion of light and color in my e-book.

The exceptional creative control of light and the great render engine in Vue 5 Infinite were some of the things what got me into Vue. And the IBL and HDRI possibilities are very exciting.

_jc  'Art Head Start' e-book: Learn digital art skills $19.95
'Art Head Start.com Free chapter, Vue tutorials, models, Web Tutorials Directory.