On weblocks-dev, the append-only semantics are naturally good. Mainline development and stable branches should only proceed in one direction.
However, some Weblocks hackers would like to manage topic branches, such as weblocks-nav4, on top of dev's tip. So that when weblocks-dev grows, they use hg 1.1's new rebase to move the topic branch content ahead.
However, with append-only, this will only succeed in creating a new head and a bunch of "duplicate" revisions.