More people wouldn't use drugs, but taking the criminal element out of obtaining those drugs may be to our benefit. When you push people to the fringe, they feel dijointed and out of sorts with their society so crimes against said society have no real worry for them. Make it okay for them to do drugs in their own homes, and make it so that the price of drugs comes down because it's no longer illegal, and crime will also come down. You can't stop drug use, but you can decriminalize it so that less people are harmed overall.
I think that people will do drugs regardless of whether or not it's legal. No more home meth labs to blow up, less high speed drug chases, less crack whores... etc. And companies will still be allowed to test and exclude people from employment for doing drugs, so there is still incentive to stay on the straight and narrow.
And people who do drugs to excess will eventually kill themselves, but once it's legal, they would be much less likely to take someone with them.