I am a doctor, I don't just play one on this forum.
All smoking is bad for you - lets just get that out of the way. Professionally and on the record - QUIT!
Cigarette smoking is very harmful, the inhaling of hot gases with carcinogens is extremely bad for your health and the number one preventable cause of death. Risks = Cancer / COPD (emphysema) / Peripheral Vascular Disease (heart attack, stroke, poor circulation, hardening of the arteries)
Nicotine is extremely addictive.
Chew is strongly linked to oral cancers.
Pipe smoking in general is much less harmful, but is dose dependent. From what I have seen in studies, occasional use has minimal risk. That is assuming that you do not inhale. If you inhale, it likely is similar to cigarettes or possibly worse. If you smoke daily your risk will increase, and if you are smoking multiple bowls every day then you are certainly addicted to nicotine and have substantial increased health risks of COPD and cancer.
IMO there will never be an honest study of pipe smoking for a couple of reasons. First, it is so overshadowed by cigarettes that any study will not apply and would not be accurate for many reasons I will not get in to here. Second, who is going to pay for or publish any study that would be favorable to any tobacco use?!! It will not happen. From what I can deduce from the scant evidence, less than daily use of pipes and not inhaling carries minimal increased risks to your health.
As others have alluded to, there may be some beneficial affects of pipe smoking for anxiety and other psychiatric ailments. There is certainly a huge benefit to people who quit cigarettes and use pipes as an alternative, assuming they do not regularly inhale or smoke all day long.
But let's not kid ourselves, just because there is a lack of evidence directly related to pipe smoking and possibly some theoretical benefits, we can't assume that it is harmless or even beneficial over all. It is in the end a behavior that is harmful to your health. As an adult you have to make the decision on what risks you want to take in life and what adds quality to your life. Do you want to be alive or live? We all take risks in life and society. Driving is extremely risky for instance, but we all accept that risk to increase the quality of our lives. Alcohol, nuff said. Tobacco remains the number 1 preventable cause of death in America. Obesity is also very bad - eating excessive calories is detrimental to your health.
Personally I accept the risk and enjoy smoking a pipe. I will smoke quite heavily some days, but many days I don't smoke at all. As long as I don't have any withdrawal symptoms when I don't smoke and it does not impact my family relations such as spending time with my grandchildren, then I feel the risk is acceptable.
I may be a doctor, but I also am a risk taker. I drive motorcycles fast without a helmet for long distances. I just finished a Texas Iron Butt ride for 1500 miles in 24 hours. So take that into account when you read the above. I am sure that most other physicians would not agree with my personal opinion and advise to quite smoking.
-Doc