Units are complicated and many people overstate the benefits of having uniform worldwide units. If I'm choosing a unit for how I sell my goods, what's more important, that the person down the street is familiar with the unit, or somebody from Ghana will be familiar if he travels to my store.
In industry, whatever tool or system you're dealing with, you're going to either use something that is either imported or exported or has to be compatible with something that is imported or exported. Thus you are guaranteed that there will be SI units somewhere in your process and it is usually just easier to go with it for the whole process, as is done in the military, NASA, and most US engineering firms. In addition to being internationally compatible, it is also a damn lot easier to use. Sure, if you use no unit but feet, pounds and seconds in your calculation there is no unit conversion that needs to be done, but as soon as you go into the range where you might think in miles or ounces, it becomes fairly difficult to reconcile intuition with units unless you do some fiddly calculations. Whereas a native SI user knows intuitively how long a Km and mm is in the same way an American might recon a mile or an inch.
So you may say: "why don't I buy a 2 pounds of apples, then walk a mile to work where I use SI to design parts and trajectories and what not?" Problem is, if you're thinking in non SI, then non SI units tend to sneak into where they don't belong. The Mars Climate Orbiter for example fell out of the sky because Lockheed used pound-seconds instead of newton-seconds in a calculation.
Considering how much success other countries have had switching, I'm always surprised at America's feeble efforts to do so. I think it is just something to do with Americans natural paranoia about as you say a "New World Order" or whatever else that prevents it.
I think the legislators are all dumb, and think that the population is as dumb as they themselves are. Start with temperature. For 2 years mention both Celcius and Faharenheit, and after 2 years (or even 3) drop Faharenheit. Thats what Canada did. We transitioned to metric long ago, and thats helped Honda, Toyota and non-American car manufacturers to get a foothold in Canada and elsewhere. We have tools designed for metric measurements, not tools designed to American standards.
We do keep certain traditional sizes, such as a 4'x8' panels, and 2x4 inch lumber. But the houses are constructed with Metric measure. And new construction is done with prefabricated panels, measured and machined using metric measure, and with the panels fitting together precisely.