The banning of liquids and pastes are logical. There are some extremely volatile liquid and plastic explosives that would take your leg off if you dropped [1][2]. Or if they were used as primary explosives to ignite a more powerful secondary explosive. That could be disastrous if TSA allowed liquids in larger quantities.
As for the detection technologies that are currently out on the market, all I have to say is that every single one of them can be confused and circumvented, which is why they have seemingly arbitrary rules. For example, xray technologies can easily confuse plastic explosives with a piece of cheese, which is why you have to take shit out of your bag. MMW wave technologies are confused by water substances, which is why you have to do a pat down if an anomaly is detected.
My own technology that I worked on was primarily developed as a low-cost surface technique, so if you had explosive residue on your hands or baggage our detector would alarm.
[1]http://en.wikipedia.org/wiki/Nitroglycerin [2]http://www.youtube.com/watch?v=oa6fcun3kag