In a way, this was part of my point. You can drink a small amount of alcohol (and people often do) without interfering with your mental faculties very much, whereas with something like a hallucinogen, that's really not done (as far as I know; you may know more than me about this). Basically, the usage cases aren't quite the same. You're right that alcohol does have sort of a slippery-slope effect, but that depends very much on the individual. Quite frankly, I have no idea why different people have such different responses to alcohol, but I have to wonder if the difference in response is somehow related to how people want to act while drunk, even if it's not a conscious decision to do so.
It all depend on the drugs : usually, hallucinogen are very incapacitating when you take them, but are lightly addictive. Cocaine is like alcohol in that you can stay functional when you take it, but put a heavy strain on your hearth and on your psyche. Heroin is addictive as hell and destroy your health. But pot is a minor health and psyche hazard and let you fully functional.
Paul Erdős was know to work under amphetamines, proof that all these "facts" only work on average and that an individual can stay functional under any drugs in certain circumstances. Just as someone can survive a bullet wound (doesn't mean that you must legalize shooting people randomly, but does certainly mean you shouldn't mercy kill everyone who has been shot).
On the matter of drugs, you seems to forget that rich kids and persons are a favored demographic for drugs dealers, and that drugs are pretty common among them (first hand witness here). And I'm not speaking of pot. Just think about how many stars do Cocaine.
The weird thing is that drug consumption seems to be perfectly accepted, everyone knowing who take what, until he/she get busted whereas everyone act like he never knew a thing.