It is an act of mercy much like a governor's pardon power. Just as a governor's pardon power does not destroy the law, neither does juror nullification destroy the law.
It used to bother me in much the same way it bothers you. Not any longer. I've lived long enough to see and understand the wisdom behind it. A truly free people must have the power of juror nullification.
We have all witnessed how the President's pardon power has been blatantly abused by BJ Clinton. It's clear, then, that whether the exercise of jury nullification is an "act of mercy" depends entirely on the members of the jury at the time and their motives.
Clinton's pardons were certainly no act of mercy.