There's something called opportunity cost. I don't know how much money the promotion of the Day of Silence cost, and it probably isn't much, but could the money, could the time, could the enthusiasm be used better. That's something anyone who has limited resources has to consider.
Now, we don't know that it helps. For all we know it hurts - we have no less data supporting that. Does it raise consciousness? We don't know. Do people remember this the week after? We don't know.
It's been 16 years - an evaluation would be nice, otherwise we don't know whether we should be doing this or something else.