Not being a texture expert, I've seen what Nadnerb is talking about, but not in Blender (because I haven't messed with it), but in texture making programs like Genetica. They have the combine function and you see: A + B, A - B, A * B, etc.
When I've messed with it, I keep thinking that I'm adding, but in most of the cases you actually use multiply for the textures to get what you want.
Me, I just play with it until I see what I want.
