Bookmark != tab
I would think that having one concept for 'bookmark' and another concept for 'tab' would be enough reason to have a management tool for both. We've had a bookmark management tool for years, before tabs even evolved. Name them, attach descriptions, organise them into folders, and these days, sync them to remote servers and add metadata and whatnot.
Tabs evolved, but the management tools for these are evolving separately to those for bookmarks, and currently the meta-information tools relating to tabs are pretty simple. So I don't think there's any confusion going on; I believe there's a good reason to keep the concept of 'bookmark' and 'tab' separate, because of the way we use each.
Everyone likes a metaphor, so I hope this is a good one. Imagine your browser bookmarks (linking to websites) are the books on your shelf. You own them, but you're not reading them right now. Imagine your tabs (websites you have open) are books you are reading right now. I often have a couple of books on the go, and I'm reading them for different reasons (Jim recommended this one, my brother lent me this one, etc). You want to manage the books you're currently reading, which are probably scattered around your life (home, work etc) close at hand, and think about them differently to the much larger list of books on your shelf. I have many more open tabs at any time than books on the go, so a management tool would be a worthy thing to have.
Having said all this, I agree with another poster here, I just keeps tabs organised into groups by opening a new Firefox window and grouping tabs in each window. For me, it's a benefit that when I restart the browser all those tabs go away, clearing up system memory giving me a fresh start. I have 4 virtual desktops (under Linux) which makes it simple to keep entire Firefox windows, and all their tabs, organised so they don't conflict on my screen.
Things I do currently miss, having seen Tab Candy, is an ability to easily move tabs between windows (when you've started browsing news and now have 6 tabs open in the same window you had your work in and want to separate the two - you can drag each tab but I usually find this quite cumbersome), and an ability to share a browsing session with another person so they see the same tabs open that you do. Integration with an instant messaging API or social networking API would be favourite.