There's a lot of accidents every day. Some involve drink, some don't.
There are accidents where, yes, you have to feel sorry for the driver, who has had a drink, is driving perfectly, and, for example someone comes out of a side road and T bones them. It would have made no difference what the driver would have drunk, the accident would still have happened.
Also, you'll find that some drink drives, make themselves stand out because they are driving properly, instead of the more normal aim and hope of non drink drivers. Unfortunately it does make them stand out, and they end up getting nicked.
The original limit was based on what it was thought would be an average level of alcohol which was impair driving. It really is just an arbitary number though. a 17 year old who's never touched a drop before would be far more impaired than a 50 year old who's drunk 2 pints every day since he was 18. BAC concentration might well be the same.
People use the Americanism of DUI, and it really is misleading, the offence is driving while having a Blood Alcohol Concentration above the prescribed limit. Those 2 things are not the same. Nor is the offence of driving while unfit though drink or drugs.