What is my point? That there are many different factors that could influence pilot's abilities to fly safely. The most difficult thing is the impossibility to find out who is actually totally fit to do his/her responsible work and who is not. It is pretty easy to identify the alcohol intoxication (and measure it), but much harder to find an efficient marker for drug abuse, narcotics use and sleep deprivation. Then there are stresses and all kind of psychiatric disorders. It's impossible to identify it at the entrance (somebody looks weird, okay, how can you prove it that a guy has somewhat more serious problem than just a bad day?)...