Getting a blood test means either going to the police station, or letting a cop stick a needle in you on the side of the road. We should do whatever we can do avoid making innocent people do either one.
You have to go to the station anyway, the Portable BAC they use roadside is inadmissible in court, they give you a more accurate test at the station if you fail roadside. Having a nurse available to take a blood sample instead shouldn't be unreasonable.