There are something like 20 minerals that are important to US industry (some are essential) for which the US has no usable internal sources and so relies on imports. For many of these, most of the world's supply is in assorted unstable third world countries (mostly in Africa), or the former Soviet Union, or China. These minerals are often called "strategic minerals".

For instance, here's a map of where cobalt comes from: http://upload.wikimedia.org/wikipedia/en/9/94/2005cobalt_%28.... (Cobalt is essential in making the alloys used in jet engines, so a loss of access to cobalt could serious harm the US militarily).

Having an emergency backup plan for these minerals, such as moon mining, would be good even if it is expensive.