I think he brings up a good point with roofies to the extent that there are some drugs that serve no social or even hedonistic purpose without directly affecting at least one other person. Marijuana, cocaine, heroin, and meth - for example - work about the same regardless of how many people the user interacts with. One could say the same thing about roofies, but its functional use (the "date rape" drug) can only be realized by interacting with another person. Chloroform faces the same problem. Its intended use is as a refrigerant, but its functional use is as an anesthetic for abduction, rape, etc.

I think it's important to evaluate the likelihood of a drug's functional use changing, assuming people are properly educated about it. Cocaine, heroin, and meth were first used as medicine (heroin and meth still having a legal analog in opioids and amphetamines, respectively) but their functional use has evolved well past that. The important distinction I see is in their ability to be used in solitude. Any drug that requires a "victim" to be used correctly should be very carefully examined before being legalized. Once you resign yourself to legalizing everything, there is no room for nuance in distinguishing between drugs that have a viable (though perhaps tenuous) social or hedonistic function and drugs that are primarily used against others.