Frictional unemployment is very important in any economy.   This type of unemployment takes place when people leave their old jobs for new ones.   It is human nature to look for the next best thing.   With the gradual increases in cost of living individuals need to look for new jobs that pay more.   These individuals can be recent university graduates that used to work at fast food restaurants who are now ready to put their business degrees to work, to those individuals already in a job who need some time off to transition to a new job.  
Frictional unemployment can also take place if a company does not want to hire applicants because they do not believe they have received any qualified applications.   When this happens the position stays vacant until a qualified individual is found.   So, without frictional unemployment people would have to barter for their next job.   Everyone would be in a job and no one would have to look for one.   Could you imagine having to wait for someone to die to move up the ladders of success or you would actually have to have a job that both participants want so you can each swap with one another.   I do not see this as an ideal situation so, yes, it is important to have frictional unemployment in any economy.

The reason the United States has free trade between its own 50 states is because we all are on the same domestic market.   Wages within the United States are reasonably close in pay and benefits.   Since everyone uses the dollar as currency we do not have to worry about an exchange rate which helps keeps the values of the products within our boundaries all the same.   We do not have to worry about one state trying to bankrupt an industry in another state just because they are competing businesses.   Since our labor cost the same in most states this keeps an even keel on the market goods.      
If we allowed free trade amongst other countries we may become victim of dumping.   This takes place when another country sells us goods at...