If it's cheaper, that means addicts are less inclined to steal your lawn furniture to get their next fix.
If it's legal, that means addicts aren't risking overdoses or injecting rat poison into themselves. Remember all those overdoses a few months back, due to extra-potent heroin being on the market? That wouldn't happen in a legal and regulated industry where the potency is controlled and displayed for the buyer's information. Not to mention the fact that addicts are 100x more inclined to come in for help when they know they won't be arrested for their drug use.
Then there's the fact that drugs like crack, synthetic marijuana, and bath salts would not exist if not for the War on Drugs. People keep trying to find new and cheap substances that aren't specifically controlled for in the laws, and these new substances end up being much worse than the original. The War on Drugs causes more problems than it solves (which it doesn't really solve any in the first place).
The legalization and regulation of all drugs would make drug abuse safer for both addicts and the rest of society.