First and foremost, as we know smoking in crowded places is harmful not only to person who carries cigarette. All people standing around that person breathe in contaminant smoke. Let's take a bus stop as an example, there is always a lot of people there. When one individual decides to light up the cigarette, everyone around him has to breathe in nicotine from the air.
Secondly, it is strongly uncomfortable for non-smoking people to get into a bar or restaurant where cigarette smoke wafts. For instance, when family with children wants to eat out, they usually look for a place for non-smokers in order not to let children have a contact with unhealthy smoke.
Finally, if smoking was banned, there would be fewer smokers on our streets. Psychologists have proved that young people are more likely to acquire a bad habit when smoking is commonly acceptable in society. So when this prohibition was imposed, perception of people would certainly change.
On the other hand, some people say that ban on smoking outrages citizen who smokes or even marginalizes them. To my mind health is the most important value for people and above argument is not enough compelling to stop the changes.
To sum up, smoking is unhealthy for our organisms - particularly lungs. Banning it from public places will bring about only advantages. We will no more worry whether the air which we breath in the street contains the nicotine. Also the number of people who suffer from lung diseases will decrease