Alan,
I am thinking about your comment, "If it only keeps out one terrorist who would blow himself up in a crowded American marketplace, it's worth it.
If I would logically follow up on that, if one gun is used to kill we should ban all guns, if one person dies due to lack of access to healthcare we should have healthcare for everyone. There should be zero tolerance for any type of pollution, I think you see where I am going with this. By the way, I don't think we should ban all guns although the process to obtain them is woefully inadequate and I do believe there should be universal healthcare.
There is risk, I don't think anyone denies this but it must be weighed against the suffering and death of tens of thousands of people who we could help. The current vetting process is 2 years long, it is already "extreme" and none of the countries being banned by the President had people involved in 9-11. Our "local" terrorists have been in this country for years, many of them are Christian Fundamentalists or other "religious" groups. The truth is we cannot prevent every instance of terrorism or any other harmful thing, although we have been doing a pretty good job of it.
Bastardization of religion has been an excuse used for millennia to claim some moral superiority over others, making them "less human" and justifying their extinction.
My condolences on the loss of your niece, I live just outside NYC and know many people who lost loved ones or who narrowly escaped their own death.
I believe we make the world a more peaceful and safer place by reaching out a helpful hand rather than slamming a door in the face of those who need help.
Jim