I think the light placement is a major factor in your lack of visible effect, but the need for a real bumpmap applies to any application - sometimes you can get away with using the image (if the bump is irregular it will probably look OK, or if the lightness of the image is a reliable indication of height) and that will be as true in D|S as in anything else.

The Min and Max values just tell you how high/low above the basic surface black (min) and white (max) are, when the strength iss et to 100%. They have to be in some unit - either an absolute unit or a percentage of the object's size - otherwise you woudn't be able to use them.