Actually, from the Libertarian perspective, legalizing drugs frees up our officers to go after the real problem: Drug dealers. See, no, the individual stoner is likely not harming anyone. Bear in mind that there are drugs that, even by Libertarian standards, should be illegal, such as PCP, and rufies. There is no safe way to use PCP, and it can lead directly to violence against others, not by odd anecdote, but as a direct effect of the drug. Rufies are specifically for use against another person to knock them out, so again, still illegal.
The bigger issue is the dealers, because they are harming people other than themselves. With decrminalized drug use, comes the freed up manpower to focus on the pushers instead of the marks. It would also decrease prison populations significantly, which in turn, since drugs make it in to prison all the time, would actually lower the overall crime rate. Fewer ex-cons means that people who used to get busted for smoking pot or doing some ecstasy will no longer have a bullshit item on their record that can deny them employment, drop their wages, and otherwise push them in the direction of further criminal/drug-related activity.
Another thing starts to go away: Mystique. Plenty of drug use is done out of rebellion, or because it's so "edgy". It's sort of like with guns: If everyone would stop freaking out about them, and treat them like tools, we wouldn't have an issue. Because we make them so fascinating, so taboo, we actually insure their continued draw.