The problem with this is that all of those breathalyzer kits will be uncalibrated.
Units used by police are approved units that are constantly and regularly calibrated to ensure accuracy.
How will they get around the inevitable cases where someone tests themselves which shows them to be under, then they drive, get picked up and they are actually over?
It opens up a can of worms IMO.
I guess as a guide it can help, providing drivers don't take the readings as gospe.
The preliminary units used by police at the side of the road are also just a guide as to whether a driver's BAC may be over the legal limit but it is the actual Breathalyzer or Drager unit which is the one enforceable by law.