Several observations:
1. The effects from smoking tobacco are cumulative. That is, one cigarette is bad for you, two are worse, and so on. If you’re a two-pack-a-day smoker your risk is higher than a one-pack-a-day smoker, whose risk is higher than a person who smokes once or twice a day. Most marijuana smokers (and I was one, once) don’t smoke nearly as much on a daily basis as “a pack” of cigarettes. Even when I “smoked like a chimney”, it was still only a couple of times a day at the most – and usually less than that.
2. On the other hand, cigarette smokers don’t inhale to the depth (and for the length of time) that marijuana smokers do. (Someone offered me a cigarette once when I was stoned, and I smoked it like a joint, inhaling deeply and holding it. It literally knocked me down to do that, the smoke was so intense.) So there may be some added danger from marijuana smoking due to the depth of inhaling and the time held in the lungs. Still, over the course of a day, it seems to me that even a “heavy” marijuana smoker is inhaling less smoke than a regular smoker.
3. Nicotine is addictive (and poisonous) on its own. Nicotine is one of the best natural pesticides around, and is lethal to humans in moderate doses in a pure form – see the link. Marijuana creates a dependency which is much less strong than an addiction to tobacco. If you can create a ‘high’ in other ways or get away from the need for a high, then desire for marijuana is naturally lessened. Most people seem to mature out of a “need” for marijuana, even if a nostalgic urge to re-create the past sometimes occurs.
4. As to whether or not smoking marijuana will cause cancer, a good question, the question can’t even be studied by experimentation (in the USA) without specific permission from the federal government, which is nearly impossible to obtain. My own belief is that the government doesn’t want to find out how relatively harmless (that is, relatively less likely to cause harm) marijuana is than either tobacco or alcohol, because that would inevitably fuel a stronger drive to not only decriminalize it, but to legalize it outright.