You can't compare default trust list with default ignore list. Those are basically two different things except both are "default".
@dogie re. "No one would see their expanded posts (by default), and they won't necessarily know it."
Sure, just like Default Trust. Where do you see a difference?
The major function of a forum is to post, this would inhibit that. Trading/trusting another user is not the major function of the forum.
But I don't think that ignore lists should be public (unless one would like to share his own publicly) for the simple reason that they can be personal.
You can bulk edit here https://bitcointalk.org/index.php?action=profile;sa=ignprefs