The real threat of terrorism is incredibly low, On average 2 Americans per year are killed by Jihadist immigrants. The people that the ban is against. Even if you count all terrorist attacks by Jihadists, such as converted US citizens, these are just 9 per year on average. It is far more likely to be shot by a toddler (21), be killed by lightning (27), hit by a bus (264) or being shot by a fellow American (11 737). See: http://www.euronews.com/2017/01/31/armed-toddlers-kill-twice-as-many-americans-each-year-than-terrorists
As such the real threat is very low, one should worry far more about idiots with guns than terrorists. Creating such a ban does not only hardly do anything to protect Americans, it also has a very negative effect on a lot of people. First of all it makes it difficult for refugees that are suffering tremendously in war-torn countries to find refuge. Alone in Syria 400,000 people have died in the civil war and millions had to flee from their homes. Also the ban brings a lot of hardship to good people from the banned countries, who are working, studying or have family in the USA.
The USA already has some of the strictest vetting processes in the world deciding who they let in. Given these strong processes and the extremely small risk, the ban is extremely excessive at the expense of a lot of people.