What the Doonesbury comic gets to is that the advice of counsel defense is most credible when the legal analysis at issue is technical or arcane. Ignorance of the law is no excuse as a rule, but society recognizes that some laws are really difficult to understand and comply with in every particular. And in those instances, someone who made an honest effort to comply – not just by giving it the ol’ college try but by seeking advice from one apparently competent to give it – is not as culpable as someone else.
That said, it does create perverse incentives, as the “best” legal analysis in the corporate setting is always render the broadest, most loosy-goosy opinion that’s even remotely credible. That gives your client the most freedom to act in contravention of the purpose of the regulation which, by definition, means that the population at large is protected less than the law should. And so long as it’s not blatantly wrong advice, it’s not malpractice – professionals are charged with giving reasonable advice, not correct advice. Plus, who would sue you for malpractice anyway when your opinion kept the client from paying thousands in fines that he should have.
–Cliffy