http://www.skydivephiladelphia.com/wp-content/themes/asc/ima...
Even less than fully fueled that thing could put a pretty big hole in a building and the cockpit doesn't even HAVE a door separating it from the main cabin. Yes, it might prove tricky to take over a flight full of instructors and students in the air, but given that legitimate customers have easy access to the airfield at ground level before and after a jump and there's no real security there it's not hard to imagine ways a sufficiently motivated and unscrupulous person could take over such a plane at relatively minimal risk to themselves and weaponize it.
But yeah, that's just one of a zillion possible threats anybody with half a brain can come up with. The current airport security model assumes the existence of terrorists who have a really weird set of characteristics such that they are simultaneously:
(1) sufficiently driven and resourceful to successfully bring down a plane via a bomb or weapons if there were no security (this is pretty hard)
(2) NOT sufficiently driven and resourceful to find a way AROUND the current security measures (even though this is easy)
(3) also NOT sufficiently driven and resourceful as to find some OTHER way of causing a similar amount of terror and damage, such as blowing up 5 busses or funding a half-dozen "DC snipers" around the country or blowing up the security line itself.
Nevertheless, your point is important. There are a lot of potential "terrorist tools" that are not protected. How about a terrorist taking over a subway/train and running full speed until it crashes or derails? I can't be the only one who's noticed operators are almost always alone. Even during a shift/operator change, overpowering 2 of them wouldn't be difficult. Same goes for most other type of public transportation (commuter trains, busses, trolley, ferry, etc.)
There are a LOT of terrorist plot/targets that scare me more than a commercial airliner today. Lots of things that keep me awake at night and wonder about the world our kids will inherit. I'll be completely shocked if a commercial airliner type of plot happens again in my lifetime. At least in that environment people would be far more aggressive towards an attack during a flight. The US government is well behind the curve with TSA and it seems clear they are simply attempting to justify the TSA's existence.