I believe that most terrorists are nationals of the country they live in, though I’m not certain on that point. So while you’re concentrating on all the people with Saudi passports, you’re ignoring the much more dangerous guys with American passports.
Or if you concentrate on race, there’s the worry that while you’re concentrating on the guys in turbans, that all the ultra-nationalists, PETA, and other sorts of terrorists will be essentially given a free pass. You’re better to have a clear and consistent method of finding dangerous materials and people than to have a single-minded one.
But in the end it’s the results that count. The problem being that we’re talking about something that is of such minimum occurrence that creating statistically meaningful test runs would be nearly impossible.
Ultimately, there was decades of time where terrorism wasn’t a worry. Even now I’m not sure that it really is, probably because the Atlantic ocean and minimal social security plans for non-hardworking foreigners make it hard for potential terrorists to get here, hunker down, and plot. Crime flows like water, down the path of least resistance. That path leads to Europe not to the US. 9/11 was most likely a statistical anomaly caused by one man who actually had a vision, rather than simply being happy to be the powerful head of a cult. That no further events have occurred indicates that that man has been captured or killed. Minus him, it’s unlikely that further significant attacks will occur.
Compare the IRA previous to and after Michael Collins. Only when he was there coming up with strategy and actually telling his men to go out and accomplish things were things getting getting accomplished. Randomly blowing up things on a whim occasionally isn’t forging a war, it’s satisfying your ego and you just don’t have to go out of your way to do that. Hitting the US requires going out of your way. If you get rid of the guy who actually views it as a war, you’re free for the rest of time. If it was just an act on a whim, then everything will return to how it was before because it was a statistically unlikely act.
That isn’t to say that you want to ignore the threat, since if you dangle your ass out, the other guy is going to be forced by the expectations of his underlings to go out and kick it. But the amount of extra effort you expend only needs to be enough to give him an excuse to be lazy.