The problem with the metric system and the reason why the US will never convert to it is it's not natural (for us). Although it may be easier we would still have to convert between the two systems because we can not see metric quantities in our mind's eye. As the member above has stated, 28 degrees F to an American is well below freezing. We automatically know this. And although most of us can easily convert 28 degrees F to celsius, it doesn't have an automatic reaction to temperature that comes instinctively when you hear someone say 28 degrees F.
This can be overcome by teaching it to the newest generation of ******** at the very beginning of their school enrollment, but you still can not overcome the problem of the 3 or 4 generations that are accustom to the current system. Because in order for it to be natural, it has to be the only system you teach to the new generation, but you still have the old generation using a different system, which wouldn't work. And if you try to teach the new generation both systems, the metric system will never become natural because other than in school, they will never use it, since the predominant number of citizens will still be using the current system. Hopefully, that made some sense.
It's a catch-22 that can not be overcome. Converting to the metric system may be convenient to scientist and other countries, but your average Joe does not want to take the time and try and convert length, distance, weight, etc. when there is a perfectly fine system in place and there is no legitimate reason to show why it is necessary to do so.