By the same token, one could say that age is not a good gauge of maturity and the ability to handle responsibility--some kids could handle the responsibility and control of a vehicle as young as 14, while some adults should not be driving into their thirties.
I think the law as written was based on some scientific data and averaged out (I can't prove this--it's just a theory) that the average person when their BAC was .08% will be impaired in driving enough to be a serious hazard to most people. I think the fact that it's not at exactly 0 is because some people have the nearest bar or other drinking venue so far away.
Personally, i go by the following: In your average drink (pint, glass of wine, shot), it takes the average human one hour to burn off the alcohol. As such, I time from my last drink in hours how many I've had, and then drive home only after that. So, if on a usual night drinking, I have a couple of pints, I'll wait two hours after I stop drinking before I head home. More, if the drink is a multi-shot cocktail, an extra large beer, or a beer with a higher alcohol content (a extra-large beer with a higher alcohol content I'll just count as 2). It makes life simpler to deal with.
That, or I simply won't drink.
I think the issue though is clouded by the precept that driving is a privilege, not a right. I think the law could care less if you can't get to work otherwise; their stance is that if you know it's your only route to work then you should be adult enough to know your limits and not put your livelihood in jeopardy by driving drunk.