Yes, a national missile defense system is a great idea for the United States, because no nation can complain about the United States wanting to play defense militarily. The United States only has Mexico and Canada nearby, so other nations cannot really complain that the United States will use their weapons offensively.
I totally disagree, it would take a lot of money from our taxes, this money should be used in better ways instead of building something that we may never end up using or maybe would be to obsolete to use in that case. We should better be resolving conflicts with other entities and/or countries instead of arming ourselves against something that we may never have. For example, what would you use against planes like in 9/11 ? Those missiles? That would not solve anything !