" it may well be in our authors' interest to limit pre-publicity of their results "
Frankly, I disagree on the statement (Well they can require what they want for their journal).
I am contrary to any embargo. I noticed that in Physics the free distribution of Preprints was massive even before the present era of web 2.0 with no problems whatsoever. At the same time, I believe (actually Nature would probably agree with me on this point) that I am able to judge my Science's topics with the same deepness of any other of my peers, and that getting any scientific information early is very useful and propel the advancement of science.
Granted this, it would be very difficult in the present times, to filter out any written information, from the public.
Moreover, if scientists do not make the effort to show and make to understand the public how science work, how can we believe it will support us ? If we do not give to the society a sample of what the scientific debate is, and what is about ... in the matter, how can we think our work will not be degraded to opinons ? If we do not grow people to understand the problematics of our activities, and the methods to solve them, how can we believe science can be adopted as a better way of thinking, and not just because it provides products ?
On the contrary I appreciate the EGU open access journals where the policy to show discussion papers, both improve the "preprints", and make more careful most of the peer reviews.
Wittgenstein teaches: what can be said at all can be said clearly, and what we cannot talk about we must pass over in silence.