I can't beat the argument (with video) that he presents, though I can present his outline:
Drug use is immoral!
But prohibition doesn't stop it.
Drug use is dangerous and destroying our society!
Prohibition doesn't stop it.
Drugs are illegal and drug users are criminals!
Prohibition doesn't stop these kinds of criminals.
Repealing prohibition sends a message that drug use is acceptable!
Prohibition doesn't stop drug use.
It's a funny thing about drug (and alcohol) abuse, that the user never stops until he's convinced that drug (and alcohol) abuse is unprofitable. Or that those other considerations everybody is squawking about are actually important.
He who has ears to hear, let him hear.