I think the major thing is that nearly all Americans cannot visualize a centimeter, while they can an inch.  I had plenty of exposure to using SI units all throughout school....but I still prefer to use imperial units (still like calling it Standard though).  If there is ever going to be any sort of conversion....it's going to take a VERY long time.<br/><br/>Don't think I'll ever care for their system of calling out thread pitch.  1/4"-28 or 1/4"-20 is so much simpler than 6mm x 1.25.....or 6mm x 1.5.