In a recent post in Cosmic Variance, Sean Carroll recalls his personal experiences when talking about the metric system in the US, one of only 3 countries that have not officially adopted the SI system. What really amazes me is that even in physics we have some remarkably poor decisions about units, the usual decision to measure E (the electric field) and B (the magnetic field) in different units seems to me ridiculous and is a common point of confusion among begginers (so E and B compose the electromagnetic field but each one has different units?), probably the reason for this is that the mks are much more popular in engineering but I still insist that cgs units avoid obscuring the structure of the theory. I also dislike some modern physics textbooks that in the chapter devoted special relativity never get to writing time as ct, for a nice discussion of this (and other topics in SR) look at the classic book Spacetime Physics by Taylor and Wheeler.
On practical issues it just seems that not adopting an almost worldwide systems of units is more of a hassle than just adopting it, specially because many units are similar (the meter is just a bit larger than a yard and a pound is almost half kilogram). There have been some remarkable incidents in this regard that are noteworthy: In 1983 a Boeing 767 jet ran out of fuel at 12 000 m (40,000 feet) with 61 passengers aboard, this was caused by a units issue, this is the origin of the Gimli Glider expression in western Canada which means making a spectacular foul-up, also the US$ 125 million Mars Climate Orbiter was destroyed by a metric mixup.
Finally, there is article about this metrification issue in Wikipedia where you can read the different approaches to this problem in many countries, probably US can follow the example of Ireland in this regard. Look at the map showing the countries that use the metric system, only the black ones don't use it.