To quote Henry Hazlitt:
"Economics is haunted by more fallacies than any other study known to man. This is no accident. The inherent difficulties of the subject would be great enough in any case, but they are multiplied a thousandfold by a factor that is insignificant in, say, physics, mathematics or medicine-the special pleading of selfish interests. While every group has certain economic interests identical with those of all groups, every group has also, as we shall see, interests antagonistic to those of all other groups. While certain public policies would in the long run benefit everybody, other policies would benefit one group only at the expense of all other groups. The group that would benefit by such policies, having such a direct interest in them, will argue for them plausibly and persistently. It will hire the best buyable minds to devote their whole time to presenting its case. And it will finally either convince the general public that its case is sound, or so befuddle it that clear thinking on the subject becomes next to impossible."
This is a major obstacle to "collaboratively and openly" deciding on the right policies (paternalistic or otherwise). If the process is open, it probably won't be collaborative: the discussion might end up dominated by those with special interests. I believe this is how the notion that tariffs might be a good idea (for economic purposes, anyway) has made it into public policy—this is one of the items Hazlitt's book goes through[1].
With regard specifically to "paternalistic" policies, one of the chapters is titled "The Assault on Saving", and I could easily imagine that people guided by Keynesian economics would recommend inserting trivial inconveniences to discourage people from saving their money while nudging them towards spending it, so as to increase aggregate demand and therefore the greater good.
[1] http://steshaw.org/economics-in-one-lesson/contents.html , see chapter 11 for tariffs and 24 for "The Assault on Saving"