This thread might have answered a question I was going to post. Let me look at it when I get home. If this solves my problem, I might actually be able to give an example of when the original behavior could be useful.
ADDED LATER:
Nope. My situation was much more basic. I forgot how I had done my UV unwrap. I purposely overlapped some of the meshes (which I rarely do), and some "similar looking" parts of my UV map were giving me surprising results when I painted it a few days later. So that means I do not have a handy example of why you would want it to behave the other way.