The adoption of the International System of Units will allow the United States to speak a common language of measurement with the rest of the world.
SI Units (metric) are already commonly used in the US by those in the fields of science and engineering. The elimination of the use of English/US customary units will further decrease the confusion caused by the teachings of both systems in public schools and universities.
For evidence of troubles caused by the confusion of units see the Mars Climate Orbiter Incident:
http://en.wikipedia.org/wiki/Mars_Climate_Orbiter
For more information on SI units:
http://en.wikipedia.org/wiki/International_System_of_Units