The biggest potential impact is reduced airflow from clogged fans and ducting which could lead to overheating. The smoke particles themselves are probably abrasive to the internal components but from my own personal experience it would have to be an incredibly slow process. I've worked on computers that were so full of smoke/ash/dust that you couldn't even see the PCB anymore. Many of these machines were 5+ years old. It's a very slow process at worst. I'd say smoking is no more of a danger to a computer than owning a pet and having your fans clogged up with their hair & dander.