When the Canadians banned it years ago it was to ensure a smoke-free environment to all of the crew. Canadian legislation required this for all workers.
With regards to the weight of nicotine etc, I've heard the opposite - that the minute smoke trails around rivets and bulkheads were the best way to detect a very slow leak. However that is not a good reason to allow it.
In the last few years of 'smoking on aircraft' I was often assigned a seat in a smoking row - (flying standby) and no-smoking seats no longer available. Often, during the flight some (expletive deleted) smoker who had a non-smoking seat, would come back and sit and smoke next to me. Rarely would they ask if was OK. Bar-Stewards!