Only reason the U.S. is taking in the refugees is because Germany and other European nations have put pressure on them by taking the initiative. It would be hypocritical of the so called leader of the free world not to follow suit. The reason Republicans don't want them in is simply due to politics imo.
The Timothy McVeighs and Dylan Roofs of the U.S. are more of a threat to national security than any refugee escaping the threat of Isis. If the Isis operatives wanted to slip through the cracks they are better off coming through the porous Mexican border where they won't be tracked in the system as opposed to official immigration channels which take years to process and vet.