In the interviews of "experts" in the security field on Fox today regarding the danger of an attack here, a couple of them opined that the hard part would be getting into the country, but we have many soft targets after that. I almost fell off the sofa to hear any so-called "expert" say that it would be hard to get into the country when caravans of illegals enter over the Mexican border day and night. We are doomed.
It is definitely not that hard to cross the border... Too many achieve their goal to say otherwise.
As far as Al Qaeda using the borders - my guess is that these people are the "expendable" type of operatives. IMHO we have to start breaking up the cells of operatives who have been here legally for years - some of whom are now citizens - if we want to end the threat from Al Qaeda.
I will admit that closing the border might at least help in regards to making it harder for terrorists - and other illegal immigrants - to take advantage of the US.