I am far from an expert but I would think smoking anything has to be bad for your health so smoking pot may in fact lead to more people getting cancer.
I think many people want to defend pot on the grounds that it can help those that are sick feel better in some cases, many of those that defend it the loudest though have no intention of using it because they are sick they just want to have a good time.
"pot" has a very large number of delivery methods, many of which neither require or even strongly imply inhalation of smoke. Because pot use is not causal of (necessary and sufficient to require) smoke inhalation, and because smoke inhalation is the causal agent of the cancer in question, pot use is not causal of cancer.
This is a loaded question. Does pot cause cancer? No. In fact, the drug in pot that gets you high has been proven to be harmless. The act of smoking, however, can cause (primarily lung) cancer. Smoking legal herbs, has the same effect though. You couldn't even change the question to either "Can smoking cause cancer" or "Does THC cause cancer" because there is definitive answers for both these questions.
Marijuana itself does not cause cancer. It can actually help kill cancer cells . The method in which marijuana is used is what can lead to cancers. If smoked, lung damage can occur over time depending on method, and if tobacco is used, that can cause cancer. In itself though, marijuana and THC are not cancer causing constituents.