I'd have to agree with p-tests for people that have jobs where  there mistakes can cause themselves, or others around them injury. 
Pilots, bus drivers, machine operators, cops, firemen, ect. 
If i were the employer, why should i chance that the guy smoked up 4 days ago, or 4 hours ago. Besides pot, the drug tests are looking for other things as well. 
So what if you've got a school bus driver that is still blasted out of his mind  from shooting up coke all weekend. He goes to work on monday and has a seizure  while behind the wheel and wrecks, killing a bunch of kids? 
I definitely am  for any welfare, public housing, or unemployment recipients having to get p tested.