Example: if I was caught drunk driving and blew a 0.15, my lawyer shouldn't be able to say "How do you know my client didn't just use mouthwash? Mouthwash has a level of alcohol in it." If they know that's not what happened, it shouldn't be legal to plant in the mind of a jury or judge. Rather they could say factually, "How accurate are these breath tests?" or "Are you sure you administered the test correctly?" to the officer being witnessed.