Personally, I believe that in an ideal world, jury nullification would be a serious evil. However, our world has some massively screwed up laws, and refusing to enforce a bad law is a virtue.
Jury nullification is meant to protect people from "screwed up" laws, abusive laws, and these types of loopholes. I think it is meant to give an error-prone human component to it. I think jurors should be able to challenge the morals of a law. I think I agree with you on reverse jury-nullification though, this is like passing a retroactive law that throws people in jail.