It may be true, someone who is drunk may drive home safely, however, as probability would have it, one day they may have an accident.
In a big city, like back east, it's easy to either walk home or get a short taxi ride. Out West, we've got major suburban sprawl. Couple that with zoning laws and now you've got to drive to get where you're going. Someone who's had a few too many might like to take a bus, but there isn't any to take, and the taxi will cost a small fortune!!
I suppose more arrests have occurred in California since they lowered the DUI BAC (Blood Alcohol Content) to .08%, from the old limit of .10%. I just heard today that after January 1st, in California, if you kill someone and get a DUI, you'll be charged with Murder.
Regarding this topic thread, it would seem to be reasonable, that operating machinery while under the influence of any drug (or alcohol) can impair your ability to control that machinery. As such, there should be restrictions on how drunk you can be.
That being said, should you be under the influence and drive OK, you probably won't get stopped, and therefore you're practically legal. It's those little things that the cop looks for that will get the arrest (ie. NO tail light, NO headlight, NO brake light, even NO license plate light). Once you're stopped they'll always ask about any drinking, try and smell any alcohol coming from the car, and even look at your eye movement (for alcohol intoxication).
FF