But, I wouldn't call it an incremental upgrade, either - in the case of the 50L, it's just that the 'upgrade' you're getting isn't something that is readily measured in a test and plotted on a graph, you're paying for the smooth, creamy bokeh that the 50/1.4 just cannot deliver.
I suppose if you make your living shooting wide open, and you need that creamy bokeh, then it probably makes sense to go with the 50mm f/1.2 at 3x the price of the 50mm f/1.4. But for the rest of us, that extra money could be better used elsewhere. By all accounts, the 50mm f/1.4 is actually the sharper lens after about f/2.8. If that's where you shoot, and don't go for the wide open apertures, well it's your money.