People like to say "we should screen Muslim extremists" or "people from countries x, y, and z" as if that information is printed on the boarding pass.

Let's not forget the Muslim world spans far beyond the Middle East. Extreme Islam spans to northern Africa, south Asia, and now even western Europe. A person's citizenship is relatively inconclusive...an Indian citizen might "look" more like our American version of "what a terrorist looks like", even though he may be Hindu, while the French citizen of mixed ancestry actually holds the extreme belief.

Profiling "middle eastern looking people" only serves to guarantee that terrorists would choose someone of Asian, African, or European descent in a terrorist act, or choose plant an explosive device in grandma's walker or baby's stroller (because THEY can't be terrorists...right?)...or even easier than that, in the cargo hold.