Molecules like benzo[a]pyrene are produced in any combustion of organic material in the real world (not under meticulously controlled conditions). It occurs because combustion of the organic material is incomplete. Since cigarettes burn slowly, with only small exposure to oxygen, you are ensured that there will be incomplete combustion of the organic material. If you took out all the things that could form a carcinogen when combusted, you'd have nothing left to smoke.
Benzo[a]pyrene is a potent carcinogen, so you're pretyt much screwed with any kind of smoking. A non-filtered cigarette contains 47 nanograms of benzo[a]pyrene, while a filtered cigarette will deliver about 20 nanograms. That's a start, but that's still a bunch of carcinogen. The real problem with cigarettes is that when you smoke you are delivering a very concentrated amount of carcinogens to your body, and it's not just one particular carcinogen, but a whole host of different carcinogens.
Smoking marijuana would not avoid any of the carcinogens from combustion. The idea that marijuana smoke is less toxic than cigarette smoke is unfounded (with the exception of nicotine content).
I don't know if there are other chemicals in cigarettes that give "pleasure" to the smoker, but nicotine is the one that is always singled out. Nicotine was once used as a pesticide (that's why the plant makes it too), can you name any other pesticide you would knowingly put into your body?
The data on benzo[a]pyrene is from Science, 178, 1197 (1972).