We understand the system we have. Why change it? Those products that have changed, I think, have done so as a means of duping their customers into thinking they're getting more than they really are. When they changed from quarts to liters, they never lowered the price to reflect the lesser volume. In their heads, people think of the one-liter or two-liter bottle of soda as being 1-2 quarts. If we need an exact measurement of something, we can use metric because pretty much everyone understands that, too. Although their heads might think in ounces and quarts, most people can calculate in metric when they need to. It's not that complicated.