I've been playing through the game for the first time, and restarting the level when I get "detected." I'm not actually seeing any reason to do this, I just had the impression that being detected equals some kind of failure, and warrants a restart. Am I wasting my time? More importantly, is there some kind of fun I'm missing out on?
Apart from a few PEC challenges and the added challenge/satisfaction of completing a mission without being detected, you are not missing out on anything.