While this case seems to hinge on 0.05 or 0.08, there would be very few cases such as this - as far as I can ascertain from published results of breath test campaigns, there are very few that fall between these figure - nearly all drivers tested are way under 0.05 or way over 0.08.
A possible issue with setting the limit at zero is that as detection equipment improves, it is likely that readings above 0.0 will appear where the driver may have inadvertently breathed in small quantities of ethanol used as a solvent in various toiletries and external medication, or other incidental occurrences. For this reason, as specific non-zero threshold, for example 0.0001 would be preferred.
If you compare the statistics for over the limit drivers between those involved in accidents and those measured in random breath tests, it is clear that being over 0.05 means the driver is anywhere from twenty to fifty times more likely to have an accident. This is by far the most important known risk factor, way above experience, age, vehicle type etc, even whether disqualified, although I suspect that as drug detection methods improve, similar risk factors will show up for other drugs.
