Do you believe this is applied to every ignorance case? What about the law's that change without your knowledge, and still break it? Or what if you mis-interpret law's? Do you think those argument's are with merit on the court, without a jury trial? (Juries tend to be sympathetic and times, and non sympathetic, which depend's on cases, so there never a trust able thing) I believe most ignorance in law, is from mis-interpreting law's to mean something (Not on purpose, but rather by mistake)
This is of-course if the defendant's argument is reasonable, and true. Justice is never 100% right, but beyond a reasonable sense that the defendant made a mistake in ignorance, would it still work?