I notice a difference if you have a background set. (Pick scene from the instances list and go from there.) Then it picks up light from the background color. If the background is left alone or set to a really dark color, then ambience doesn't pick up much at all. What it mostly seems to do is affect how shadows appear by picking up scattered light. Otherwise you just get really contrasty pics in regards to light and shadow.

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Barbequed Pixels?

Your friendly neighborhood Wings3D nut.
Also feel free to browse my freebies at ShareCG.
There might be something worth downloading.