Tabs would be a useless feature, if the window manager did a better job at what they should do. In a "tabbing" wm like the one BeOS has you are much better served by one-page-one-window application.
With a bit of cooperation from the window manager you can get rid of tabs. I did it few years ago and my browsing behavior and habits have changed for the better.
Tree-style tabs (Firefox extension) are an even better solution: each new page becomes the root of a "tree", and the tabs that are opened from it become "branches". There is also a very interesting contrast between following a link (so the previous page is available on the Back button, and therefore either accessed right away or soon forgotten) versus opening it in a new tab (so the parent remains visible as a remainder of an interesting place or a branching point in the navigation).
Since we don't use desktop applications this way, I don't really see why we should modify our existing desktop window managers to accommodate this style of navigation. If anything, we could think of doing it the other way around and fitting desktop applications into a browser-like navigation structure like the one described above.
I disagree: if I could I would do all these things also in desktop applications. Web makes it easy to do, but that does not mean that we do not have the same needs in other applications.
I explore several paths also in my email application: indeed I open messages and thread in separate windows, now that Thunderbird allows me.
I branch and backtrack my terminal: I often open a new windows just to run a command and see its output, then I close it and go back to the original window. If my window manager could track that and visualize it for longer-lived shells it would be great.
I keep many applications application open as remander, yet I would like to close them if I knew that there were a simple way to recall them, or if the system would remind me about them.
I would love to bookmark application statuses and store them.
All these needs are present in the web and would be nice to have in desktop applications. My point is that tabs are not required to support these needs, a good windowing manager and some synergy with apps is all is needed.
Hyperlinks on the other hand are easily tracked, moved, rewind, etc.
Tree-style tabs is something the desktop manager should provide. And autosave for all documents like SublimeText does.
Whatever we do with the tabs in our applications is up to us.
Honestly, we don't know what the limits are until we try it.
All I get from your argument is: "we have not tried it, therefore it will fail".
I can't even operate without tree-style tabs anymore. They make it really easy to organize my thoughts, activities, and the current state of any investigation that I'm on.
They tend to start with a search, then all interesting links opened as leaves, then pruning happens, then finding the most comprehensive leaf and promoting it first leaf, then deleting the (search) root to make the most general leaf the new root.
Some leaves then end up becoming branches as I move leaves beneath them, and I may find old tabs or trees than may get moved into this tree. The final leaves usually end up being the most concrete pages - manual pages, product listings, specific vendor options, etc.
I keep these open until whatever I was working on or investigating is complete, or if other things get in the way, I can archive them by bookmarking the entire tree and giving it an appropriate folder name.
I get the suggestion that window managers could handle this, but I don't know of any window managers that handle tree-style tabs and bookmarks like that, and anything else at this point feels like a step back.
For Lightspeed's target audience, merging the UI for tabs and bookmarks might make sense. The user's mental model for both is "that website I was looking at earlier", where earlier might be a long time ago (a bookmark) or recently (a tab).
It'd also make Windows 8 / Metro usable (along with a few other tweaks to make closing the metro-crap easier).