I seem to remember it had something to do with winning cheap points on surfaces that were the higher ranked players weekness. eg top 50 player, but very week on clay would often lose to 250 ranked who is strong on clay.
Murray - sure, it doesn't make so much sense at WTA level, but in ITFs it would seem to be a good idea - I mean in 10ks you often get 1000s v 400s, and I think that win should be rewarded a little more.