I think it depends on the position. For positions where you're required to carry a firearm, or perform really technical skills, or life saving skills, then yes, drug testing should be required. It really covers the asses of the employers more than the employees, because of plausible deny-ability. If something were to get screwed up, and someone were to die, the hiring agency would be in such deep shit for hiring someone who tested positive for assumed illegal drugs in their system. There's really no way to tell WHEN exactly the drugs were consumed, unless you perform a hair test, and even then, you can only get a vague answer based on "they did drugs within the past 30, 60, 90, 180 days," and can't tell you if they did anything ON the job.

As for general bullshit office jobs that really don't alter anything, I don't see why testing would be necessary. Some office jobs are so boring that you'd have to be blazed to make it through the day. Unfortunately, I wasn't