It eliminates ambiguity (if someone says "miles", do they mean US statute miles? Nautical miles? Imperial miles? Something else?) and the fact that the units are (almost) all based on the decimal...
Because SI units are globally recognised (with the exception of Burma, Liberia and USA). SI stands for 'System International' See the related link to WIkipedia for more information.
Because the rest of the world uses the metric system. If you leave the U.S., science is still science. If we used our system, it would be hard to convert our discoveries.