Drug testing weeds out a huge amount of the population from many jobs. Personally, I think most of that is crap. I don't give a damn what folks do on the weekend or after hours, as long as they are ready for work when they get there.
I’d say depends on the drug.
I would not want to hire people in a job that requires responsibility of some kind if they were using heroin or other opioids (except with a prescription); cocaine, crack; methamphetamines; bath salts (is there a test for that?); PCP or LSD. Also, glue sniffers and huffers.
Pot or booze I wouldn’t care about, as long as we can be certain no one is stoned or drunk on duty.