Imagine you run into a situation where you have a separate geometry for left arm and right arm. You have done all the skinning for the left part and now you run into a situation, a confusion ~ "How do I copy the skin over or mirror the weights to the right arm!?"
Well the answer is simple! Make sure you have both the separate geometry bound to joints. Select both of the geometry together and just perform the "Mirror Skin Weights" operation the same way you would have done for a single geometry! It works right? ;)