No serious drug addict could finance a drug habit with welfare money. That's nowhere near enough money for all of those expensive drugs. They would have to get money for their drugs from other, less legal means, and, if the welfare money were gone, they might not miss it. People who don't take drugs and who need the assistance might get offended if they are told to pee in a cup. Given the lack of jobs (the unemployment rate has stayed above nine percent for some time), these folks would be the ones to suffer.
No drug testing just for the sake of humiliating people for being poor. It's a bad idea.