Like mentioned before "driving under the influence" and "driving while intoxicated" are two separate issues. If you want to have a hard limit for driving under the influence, which doesn't represent your physical ability to drive well or not, that is fine and it is an arbitrary number. It should be a fine, and it should trigger road-side testing. If you are "impaired" through a series of road-side test you are driving while intoxicated. Conversely, if you are under the limit, but seem intoxicated, it doesn't matter what your BAC is, the same road-side tests should be involved.
I work in the ER, I have seen people with surprisingly high BAC who are quite functional. Those people are chronically intoxicated, and have adjusted appropriately. Same goes true with the current narcotic epidemic. I have seen people on chronic narcotics who are quite functional and seem surprised when I tell them they shouldn't be driving on their 80 mg twice daily of long acting narcotic and 15 mg every 4 hours of a short acting narcotic.