Typical roadside walk the line test might not be all that objective, as it's judged by the eye of the officer, and some people may be better walking the line, while still having reduced reflexes and being a increased danger on the road.

Though, I'm not an expert and despite this feeling arbitrary, there may have been studies comparing effectiveness of various forms of testing on predicting the harm of DUI, or comapring breathalizer with the walk the line test, or touch your nose test or whatever.

I'm not for locking people up, but I also prefer safer roads. And what helps and what does not may not be all that obvious on the larger more aggregate level.