The advantage of changeset evolution is obvious to me. But as a longtime Mercurial user, I don't grok the benefit of bookmarks. Do you have a simple example where a bookmark is more useful than a permanent branch?
The teams I work with don't like the permanence of mercurial named branches. Bookmarks are a nice lightweight alternative way to name a short-lived head rather than creating a permanent tag for a two-line commit. Before bookmarks we used anonymous heads.