I asked that question for myself, and the answer (for me) is because of the hassle of switching browsers.
I switch browsers somewhat often, and I'd like to have all my bookmarks in the same place (not to mention the terror of migrating all bookmarks when getting new laptop). That's the main reason.
And of course there are little reasons such as:
* native bookmark doesn't give me preview or little snippet of what the link is about.
* I want to replay Freddie Mercury songs on YouTube in loop.
* It's always a pain to download photos of myself, tagged by family on Facebook.
Firefox sync means I don't have to worry about losing my bookmarks, and I doubt I'm going to use anything but Firefox for a long while. Your other points are interesting, but I don't really see how they beat the awesomebar integration Firefox has.
Firefox 4 + Sync (Slurp to get the bookmarks out of delicious). Additionally, use Dropbox or SugarSync to backup bookmarks folder. With 5 copies of backed up bookmarks, you'll be covered from all sides.
