To let in people, in the U.S have it's pro's and con's you have to look at the circumstances in which this country already lies. If these people are not posing a threat, or making things difficult than there is no reason why they shouldn't have a Visa, but if there is a threat to the U.S than we must protect at all cost.
The Obama administration has notified Iran that it would not grant a visa to Hamid Aboutalebi to enter the United States because of his involvement in the 1979 U.S. Embassy hostage crisis in Tehran. The United States owes loyalty to U.S. citizens above any perceived slight to Iran, and Iran should be willing to understand its position and appoint someone that has not acted in a hostile plot against the U.S.
The United States should be allowed to ban a foreign ambassador from entering the country if they pose a threat to national security or if they have committed acts against the United States in the past. Considering the Iranian ambassador played a role in the hostage crisis in 1979, I agree with the decision to block his entrance.
Our government should be allowed to block whoever we want from coming into this country, especially when safety is an issue. Other countries should be free to choose who can enter their countries as well. We should not let other countries dictate who we allow in our country since it could be such a safety issue.
If you have a problem with a country whats the point of not letting their ambassadors in? How is that solving anything?
You should be working towards a solution, not working against a solution by not negotiating. Discuss your problems, not let them sit there in hopes they will go away, or listen in this case.