While i'm all up for anything that stops or deters drink drivers, its just fraught with too many logistical issues for it to be workable - especially in metropolitan areas.
I actually talking about this with a close family friend (who also happens to be a policewoman) last nite. Apart from the convoluted process to actually obtain proof that the driver was over .05, the biggest problem she saw was actually finding and catching the drivers.
Within inner city areas, what station has the available police officers to go chasing around the back streets to find the dobbed in drunk driver? While i'm sure you
could charge someone for the offence of drink driving after they had made it home, but its just too much of a grey area for it to be tested on a day to day basis.
All sorts of issues start to arise if you are driving a car that is not your own, like all of us, i've driven friends cars home who are not fit to drive. A random, bartender or someone we didn't like, could very easily dob us in as we leave the pub, the police then spend the next hour trying to find my mates car only to find out that I, stone cold sober (don't laugh it
does happen

refused to let her drive, deciding to go back to my place to crash.
Within the country it would be a different matter entirely. My dad lives on the farm and has lost his licence due to drink driving about three years ago. (Stupid i know

) Now that he's got his licence back, if the police see his car outside the pub they will sit around the corner waiting till he comes out. Many country police do this with anyone they know to be a drink driver. The barman at Dad's local regularly reminds his patrons that the cops do sit around the corner (and along the back way too).
This is a great thing, never does my dad or any of his equally alcoholic mates drink and drive anymore, they either get their partners to pick them up or take turns to drive. A couple of months ago the barman called the cops on a guy who was on the verge of passing out, who was hell bent on driving home. This was after the barman had wrestled the guy for his keys to no success. Everyone in the area
knows they will be caught if they drink drive. And with the horrific percentage of road accidents/ fatalities in the country, not too many people have an issue with it.
The country police are also better equipped to implement an informal ' dob in a drink driver' plan. Generally less stressed for resources, they know where many people live (esp. the drunk drivers) and they have a much greater chance of catching the driver before they get home (given the geography). Also, any potential drain on resources is far more justifiable given the higher the country road toll.
But within the city, you'd just have too many people abusing the system and to actually inforce it effectively, would be a nightmare.