I have no idea why people put the diffuse image maps that are used with trans maps on a white background either, It always leaves a fringe.

Example, from Yamato's tsuta for A3... Before on a white background, It was showing a fringe of white in render (not to mention the color was too garish).

After compositing the image over a sympathetic color scheme and some tweaking of hue and saturation the results were much better. No fringe and a much more natural looking result.