The point is that scatter plots are harder to read than histograms. It's not two histograms vs. one scatter plot, it's replacing the current single histogram system with a single scatter plot.

Also, side-by-side histograms aren't the only way to display two parameters in a single histogram. What about stacked histograms? They scale up to an arbitrary number of parameters and everybody who can read a histogram can read a stacked histogram. Scatter plots seem like thermonuclear overkill for a problem which most movie sites seem to consider to be "solved".