Gingergenium made some very valid positive points on what legalising all drugs can create on the Stevens thread.
You would take the majority of crime away from it's production and trafficing (but not from the end user who's addicted and needs cash to support their addiction). The people producing heroin, cannabis etc. could be properly regulated, money going to the governments of the producing countries. In this country, the substances could be heavily taxed, like tobbacco and alcohol is, funding the clinics, hospital and therapy expenses.
The price of the substances could be reduced (though this is probably a bad thing, as it would just mean people using more) as it's no longer illegal, allowing propper trading and competition to take place.
These are all great and everything, BUT drugs like heroin, cocaine etc. are so rediculously addictive, it would never work. Legalising these drugs would send the message that it's ok to use them (look at the binge drinking culture in the UK already). It would at least send the messge that it'd be alright to try them (how most smokers start), them BAM their addicted, spending all their cash on their addiction, loosing their jobs etc. When they're out of money, they'll get some in other means, stealing.
Let's face it, these class a drugs are not the same as tobbacco and alcohol, and some can be very dangerous and mind estroying. I took magic shrooms a few times in uni a few years back (back then, you could buy them legally in a shop, it was just illegal to consume), the first couple of times were great, had great fun. But one night I had the worst experience of my life, started off ok, them I began thinking about fate and deciding if it was real, everything I did was pre-determined, and it wasn't my fault. I promptly started spitting on the floor (I was in someone elses flat), chewing on random things (pack of rizlas etc.), then things just got really scary and black. It's something I never want to go through again, and means I'd never touch the things again, no matter how good it was the first few times!
Just my 2p and an experience I've had.