They are not overly obvious (uniforms) because of a desire to lower the threat level to the average person, while still being readily observable to a trained observer. That's why they don't need to be there on every flight, just enough to make planning of "another 9/11" something that will not be done. As I stated, just like police patrols, the purpose is to deter. (Police cars are very inefficient for catching criminals - they are there to deter crime.)