Hi Carodan. I did some more tests and here is the result:

1. For busy outdoor scenes in open sunlight, a single light can be enough.

2.  If you use IBL but don't want to use IDL or AO for whatever reason, it's possible to just crank up the shadows of the main "sun" light to 110 or 120%.

3. For empty scenes or indoor scenes, a single light is too harsh as it creates a dark shadow border around the figure.

Attached is my default light setup:

First shows my OpenGL preview created by eight infinite lights. They don't render.

Then the rendered scene using an IBL fill and a point light "sun".

Then the same scene without an IBL fill. I had to crank up the "sun" to 110 % to get the same lightness as the IBL + "sun" and reduce the shadows from 750% to 500%.

Now, the result is not bad for an outdoor feel, but I really like the softness I can get from the IBL fill better.