That's odd seeing as the Brix/Plato scale was invented as a way to make SG intuitively meaningful to brewers - http://en.wikipedia.org/wiki/Brix
In short, if your wort is 12*P, it's 12% sugar by weight. That gives most people a lot better idea of what's going on than knowing that it's 1.048 times heavier than water.
Good point. I still like the units that makes mathematical sense.