In short, these are deterrents designed to prevent incidents from occurring.
In some cases, people know very well that they won't be charged if their blood alcohol levels are less than 0.10%. There are also cases in which a person involved in an accident leaves the scene of that accident, goes home and has a few drinks, then calls the police station. That persons gets off; he can't be charged. That's another problem. If there was an automatic charge—if someone refuses to take the breathalyzer, he's charged with refusing to do so, but, when someone leaves the scene of an accident, goes home, calls the police station and says he had a drink at home, he's charged with nothing. Even if you charge him with leaving the scene of an accident, he'll cite all possible reasons, such as that he was under great stress, and will get off.
Currently, there are loopholes; that's for sure. However, if it is clear to a person that he's going to be sentenced, will have to use an ignition interlock with a breathalyzer and may have his licence suspended, he'll think twice about it, especially if he has a job. With regard to prevention as well, we've discussed at length the importance of clearly indicating the consequences people who are impaired will have to face if they are in an accident or are stopped. Fear of being stopped may at times be the beginning of wisdom.
I think there are things I didn't do when I was young because I was afraid my father would catch me. Someone who is afraid of being stopped on the road may automatically designate a driver for the evening, as I see my daughters and a number of other people do. Every time they go to a discotheque, they designate a driver, who doesn't drink for the evening. Everyone takes turn; they play that role.