Why should idle threats not be taken seriously? How can you define an idle threat from a real threat? Where is that line drawn? Why should their existence be downplayed? Like I said before I'd rather them be overestimated than underestimated.
Take them seriously by defending the homeland.
If there was evidence would you feel differently? At what point before 9/11 was Al-Qaeda worth going after? Is it fair to say that ISIS is determined on their goals? They certainly seem full of zeal at the moment.
Not really. I don't see the purpose of "going to war" with non-state entities. Just improve security at home