Trump's ban on Muslims
It was a suggestion for a temporary ban while vetting process is reviewed. Your question was loaded and not objective. Not that I like the dude, but be fair.
1. PamGH hit the nail on the head.
2. I'm really not sure we should take any of Trump's "policy" statements seriously. He changes his mind, and makes these off the cuff claims as a "speak first, get elected, make policy later" kind of strategy. I have no idea how it actually works. Not a fan of the strategy, and you can bash Trump for not being serious / flip flopping all you want, but the dude is certainly not making hard and fast policy resolves when he says this stuff at his rallies.
Since we do not have a secure vetting program, a temporary halt should be instituted. We have over 20000 people we don't know where they are and no means of tracking..