At low altitudes, which is where MANPADS are used, you are correct. The bloody things are simply too fast and too smart.
Totally different story if you've got 10,000 feet between you and the ground and someone hoses off a MANPADS missile - but then, almost nobody is stupid enough to try that. Stingers have a 10,000 foot ceiling, but their kill percentage drops off notably with targets over about 5,000 feet.
I've heard that the concern is that these things have been brought across the Mexican border and are in the hands of Mexican and South American gangs. The Islamo-Nazis don't need anything but money to hire the gangsters.
Very scary.
MANPADS by necessity have very small warheads. They are more likely to go for the engines. That's not as true of the newer ones but still true. Most airlines can execute a return to base upon losing a single engine. In fact there was at least one aircraft, an Airbus A-300 from DHL, that got hit by a MANPAD SAM flying either into Baghdad, and the pilot was able to put the plane back on the runway.