Well you send the addicts to rehab, the theives to jail. The ones that lose their job, ding fries are done.
I think you're greatly underestimating how many addicts there'd be and how many people would be affected by this in a manner that would render them a burden on society. "Just leaving them alone" is not an option society seems to be capable of chosing, so there's cost.
The cost of drugs is not prohibative and the penalties do not seem to be a deterrent. The current system does, however, create a black market and make criminals out of ordinarily law abiding citizens.
There will always be something "ordinarily law abiding citizens" will do illegally. Always. Speeding, drugs, stealing cable, whatever. If it saves them money or sticks it to someone else or makes them feel outside of the system, they'll do it. It's just the way it is. There really is no such thing as a "ordinarilly law abiding citizen".
I'm not in favor of legalizing all drugs, but there is a more than strong case for legalizing marijuana.
There is absolutely not a strong case for legalizing marijuana. Once again, it is in an excellent position to be the prime illegal drug. You couldn't ask for a better drug to make illegal just because it is as harmless as people defending it make it out to be. Legalizing it, alone, would not kill society or send it into ruin, but it would set a precident that would open the door for legaling the next drug in line which is not as harmless. The arguments you can apply to marijuana legalization can be applied to virtually any drug.
There is a balance that needs to be maintained in regards to illegal activity and what can or shouldn't be tollerated. There needs to be illegal activities that can be somewhat tollerated with token gestures made to show it actually is illegal. This is actually a cornerstone of civilization... the ability to go outside of the system. Legalizing pot would make the same amount of people travel even farther to get outside of the system. The same people who don't want to get a real job right now will look for other illegal activity to survive on. This does nothing to address ciminal activity. It does nothing to bandaid society's ills.