I don't think the one size fits all 'legalise it' would work at all, I've had the same discussion many times with a friend of mine who insists that drug laws, smoking bans and such are violations of human rights, but that's obviously not the angle you're coming from. You know where I work, right by Glasgow's biggest railway station, between the business area at the west of the city centre, the retail centre bang in the middle, and the relative opulence of Merchant City in the east, and even here it's very easy to see from the people I encounter that legalising heroin for one, would not solve the real problem, which isn't necessarily people falling in to the trap of addiction (and the subsequent stigma attached to asking for help with the problem), but the mindless zombies people become when they just don't give a fuck. It's nice to envisage a world where everyone is respectable and, without things like substance abuse to drag them down, would be living as respectable citizens etc, but unfortunately there are still a minority (questionable in Glasgow at times though

) who will still recklessly abuse whether or not adequate laws and support systems are in place.
You could say that alcohol has the same effect on people but I'd say that's a better case for more restrictive alcohol laws than legalising everything else.