In an ideal world we would have one set of units for fuel, distance, whatever, but that's not going to happen - as strafer says, what's wrong with an American aircraft having gauges calibrated in US gallons?
However, one thing that I do think is daft is when an individual country cannot be consistent with itself; why does the FAA like to mix statute and nautical miles, for example? I've had IR practice questions using no fewer than four different units for distance...
