I think there is an argument for the legalisation of the use of weed but not the supply, which has actaully been shown to have some health benefits and has many simialrites to alcohol in its use, after all tobacco is legal as is alcohol and if weed could be supplied and therefore controlled it would take the profit off the street and take back control through more contact of any health issues. To be frank it's available now anyway I don't see much difference in the availability side of it.
As for the rest of them frankly I think that's an insane idea. All of them are proven to have major impacts on health and making them more available is just going to promote those issues. Not only that but the likes of heroine is used as a method of control not just by the user but by other people around the user with other aims, in prostitution for example. If it were legal it would facilitate that greatly. Some are massively addictive and once people are on it, it is very, very difficult to get them off it. So running out of money leads to illegal activities to get the next hit wether the drug is legal or not they'll still be stealing from whereever they can to fund their habit.
I don't believe it would stop the illegal supply either because to do it legally you'd have to charge the price it costs to get clean drugs officially, which means they would be expensive, which would make them cost prohibitive and mean that there would still be a market for the criminals to exploit. There would still be a market for the guns and the prostitution to gain the legal drugs so I don't see the gain there. Also those people that currently supply the illegal stuff you think they're going to going to play nice and start paying their tax and NI?....
Hard drugs are a terrible, terrible tear and a family and anything that makes it easier to get them is bad I think.
Personally I'd say legalise weed with a percentage of additional profit going to drug related issues for other types of drugs. For all the others change the way it's punished and the money is used that's recovered. Put that back into the system to help the addicts and change the minimum sentences for supply to far greater and alter the way they punish those for possession, incorporating rehab and education as well as prison.