Marijuana has usefulness - to help people with sucky things in life like multiple sclerosis, alzheimers, breast cancer and HIV. However, for people who don't need it other than to get a fix end up ruining their brains and lives because it's so addictive (so I've heard, I've never tried the stuff). It's expensive and since it's not sold commercially, no one's trying to lower their prices to get the most customers.
The point is, marijuana itself is not innately bad. It didn't ASK to be used recreationally. No. That was our doing - human tampering that ruined it all. Now it can't be legalised very easily because some awesome people went ahead and spoiled it for the rest of us who might actually need it.
The thing is, no thing, no physical object is innately bad. It's just a tool. Everything God and man made are tools. Things for us to use. It's up to us how "good" or "bad" something CAN be.
Fire is a perfect example of this. It cooks food, provides us with warmth and the early sources of light. Although, it does burn and sometimes burns down houses and cities. But it's not fire's faults or goodness that define it. It's only fair to judge it's "goodness" on a case-by-case basis. Marijuana: use it to increase appetite for HIV patients: good. Use it to feel good because nothing else in your life does: probably not good.
Everything in our lives are tools - it's up to us to give it it's value. Internet to access a worldwide network of knowledge and experiences: good. Using it to find vulnerable children for sexual gratification: not so good.
Sugar - need lot's of energy because you expect to undergo a lot of physical or mental exertion: okay. A third piece of cake: not okay. Unless you just got dumped, then honey, take the whole damn cake.
The point is, we shouldn't be so quick to categorise things, physical things, into "good" and "bad". Because things are just things. We make them good or bad. We're the problem, not the things.