I can totally see where you're coming from Stevie regarding the amount of incompetent drivers causing accidents but if you also have the same incompetent drivers drinking then they become even more of a problem hence you have to have a set pass or fail level for what qualifies as being "over the limit", I also agree that some drivers who may be just over the limit are probably still capable of driving better than some sober drivers but there is no doubt that they would drive even better if their system was totally clear of alcohol.
Also, there are laws against useless drivers namely reckless driving and driving without due care etc but the police need to make a point of enforcing these laws so yes, I agree with what you say about incompetence. In a nutshell, if all driving laws were enforced appropriately it would make us all drive better and more considerately purely to avoid picking up points or causing accidents by negligence.