I guess it begs the question of why we have a criminal offence of impaired driving or drinking and driving. It's to prevent the risk of harm that anyone who has consumed alcohol and then drives poses to other drivers. The whole idea of the “two-drink defence” or “last drink defence” is that it calls into question the reliability of the science. It says that even though this machine, which took a test some time after the offence, suggests that the person has a level of alcohol in their blood that may make them a risk, it's wrong to presume that whatever the result was an hour or an hour and a half later necessarily reflects what the person was at the time of driving. In other words, they really weren't a risk at the time they were stopped.
So it's not just some technical defence. Technical defences in the drinking-and-driving context are, for example—and it happened, and Mr. Rosenthal can tell you all about them—that an officer didn't identify the machine properly or there was something wrong with the paperwork. Admittedly that's the lifeblood of criminal law, and whether you want to do something about that is another issue, but taking away a defence that goes to whether the person really was a risk of harm and may have been factually innocent, as Mr. Rosenthal said, that we have problems with.