I personally don't think the harm is in the use, but in the abuse, and most people, will abuse marijuana sooner or later, it is just a matter of time and life circumstances.
I believe, and this is my own opinion that marijuana is a drug, addictive, and destructive. Of course not to the level of other hard drugs that literally take away your life (crack for example), but still. I think all marijuana smokers go into the same drug patterns, which start by euphoria, you know how great you felt when you smoked the first time, smart, zen, loving. And that's the hook, you start searching for that state of mind, not knowing that the more you will smoke the less you will have it. And then the habit is developed and you no more smoke to get high, but simply to get by ( you know, when you reach 3, 4 or 5 grams a day), and your reality becomes the weed reality, you see the world through the marijuana glasses. And when you don't smoke, then comes anxiety, and loss of focus.
There are certain cases, when marijuana becomes the reason of your day, as if your purpose in life is to find pot, and smoke pot. You wake up thinking of that first joint, and you go to bed after smoking the last one of the day, and in between, every occasion is a good one to roll one, 420 all day long. your time actually stops at 420. You might even hang a clock that's stuck on 420 :)

But, quitting marijuana is 10 times easier than quitting smoking, it's really not that big of a deal, but you will need a reason, you need to get to the point where you realize that you are not happy being stones all day. And you will get there, as most people do at a certain age.
Best of luck my friend.