I think you're missing the point. The idea is to make it more difficult to stage an attack around a specific location, such as a heavily populated area or landmark.
For example: http://www.af.mil/news/story.asp?id=123149266
It would be pretty easy to slip a gps antenna behind the shade before you closed it as well.
Passengers, beyond curiosity, have very little use for GPS during a flight. When I'm on a plane I could really care less what my location is. I'm more concerned with how much longer I have to wait before arriving at my destination.
The parts of the security directive that bother me most are the restrictions on access to carry-on belongings and items on your lap and not being able to get up from the seat.
Also, I could write a computer program to use my cell phone's GPS receiver to activate an electronic device at a given location. (Hell, I do this to turn off my ringtone at work.)
Stuff that into the overhead compartment along with your bomb, and the plane is finished... at exactly the location you pre-program. (You don't even need to be on the plane. Put your bags in the overhead compartment, stuff some other bags in front of it, "oh shit, I forgot $foo in the terminal", done.)
This isn't fool proof but the FAA policy that my dad experienced years ago was that the passenger was dissuaded from leaving, the captain came out, and he was told that if he exits the plane they have to sweep it and because of that he'd be denied reentry. Passenger insisted. Argued a bit.
Long story short, the passengers all had to disembark with their luggage and go back through security screening. And the flight crew presumably did their standard pre-boarding sweep.
This was, oh, 2002 or so. I wouldn't be surprised if they even let you off the plane anymore, at least not until the 3-hours are up that the new regs mandate.
And similarly, there has long been double-checks to make sure that no checked luggage is on a flight that isn't matched to a passenger. My understanding is that luggage that is freight checked (like, they lose your luggage and throw it on a later flight to get it to you) has to be more thoroughly screened.
I was flying LAN, and the first leg of the flight, in the US, was operated by American Airlines. When I got to Miami for my connection to Peru, I showed my ticked to an AA employee at the gate and asked which gate the flight would leave. She pointed me to a nearby gate, I went there, waited about an hour, and boarded when they called everyone. They manually checked my ticket and gave me the stub back.
When I reached my seat there was someone already there. He showed me his ticked stub and he had the same seat number as I did. As I was cursing the airline for overbooking, we noticed his ticket was printed by AA, mine was by LAN. Since I had just flown with AA with a LAN ticket, I thought it was OK. The flight attendant returned after a while telling me that LAN and AA both had flights to Peru at that same time and my flight was actually leaving from a completely different terminal.
I had to run all the way across the airport, get a new ticket printed, go through security again to get to the right flight. Since my bags had been checked to the final destination, if the AA flight wasn't full and they hadn't sold the seat I had I might have never noticed the mistake.
It is strange that the agents were not following the correct boarding procedure, which is to scan the barcode on your boarding pass and to ensure that the computer's idea of your seat assigument agrees with what's printed on the pass. Usually, if they disagree, a new boarding pass is printed, and you board with that. (I like when they don't match, as the next words are usually, "oh, we had to upgrade you...")
Also, by not scanning the barcode, the gate agent ensured that you would not automatically be credited with frequent-flyer miles. Highly unusual.
Now, that doesn't mean to say that you couldn't hide a bag onboard with such a device, and then make some kind of excuse that you have to go back to your office because there's some kind of emergency, and you won't be taking the flight after all. But then, you'll have had to somehow get your device into the sterile area (requiring accomplices who work there, or some surprisingly good luck getting through the x-ray scanners and explosive detection machines).
In other words, the security we had pre 9/11, with a little bit of modernizing, does the job fine. The rest of it (shoes, liquids etc) are really just theatre.
You're right that an attacker probably wouldn't back out just because the attack wouldn't be maximally effective, but the result would still be reduced casualties. Ideally, you want to prevent all attacks, but even when you can't the next best thing is to reduce the severity.
Detroit jokes aside, Metro Airport is in Romulus. It'd take a staggering amount of either skill or luck to hit even one or two buildings with plane bits. And pedestrians essentially do not exist in Michigan.
Cripes, remember when Flight 587 went down over Queens? 5 people on the ground were killed and Queens is about 4 times as densely populated as Detroit is.