I can see the FAMs being useful, if they sat in the back of the plane. Because that would catch the terrorists by surprise, no way in heck would anybody expect them to sit in coach.

If the terrorists decide to use planes again, it won't be something that FAMs could stop. Next time it will be cargo planes or some other plane that we left undefended. Heck, all they need is money to lease a few airplanes. How would a FAM stop them from doing that?