With reference to alcohol in particular, there seems to be a problem of an objective determination of a pilots level of impairment prior to flight.
"He smelled/looked/acted like he was impaired" is a recipe for huge mistakes. Something more objective is needed.
What is interesting is that this may be begging another safety question.
Going back to the basic assumption that the reason for alcohol and other restrictions is to ensure safety, could it not be argued that there is a case for examining duty limits to ensure at least an equivalent level of safety?