There are other things that lead to driving accidents, like lack of sleep or perhaps even needing to use the restroom. Ideally, a field test would cover most of these.

Someone just over the legal alcohol limit could be less impaired than a drowsy, sober driver, but the former would be more likely to be blamed for an accident than the latter, especially in areas with really low limits (e.g. it's 0.05% here in Utah).