The right half is. The recently/most visited are not available anywhere else. I felt like the repetition of the Personal, Devices and Network were good repetition: not everyone (hi!) uses the sidebar, and they are after all some of the places which it is most plausible that the user will want to go
And I agree, the tabbing experience in Gnome overall is pretty awful. You might wanna note that I added a "New tab" button in the toolbar, for discoverability. Tabs is a killer feature for web browsers, and can be for file browsers aswell, if they just get a bit of focus from developers.
Why not keep the sidebar, and just have the two other columns in the browsing window? I agree the new tab button could be useful. What we really need is a standard that is simple, beautiful, and easy for even the most computer illiterate can use.
The main problem with that approach is, as said, that not everyone is even using the sidebar to begin with. Also, there is no logical reason to put a visual divider between the zeitgeist-related lists and the ones that exist in the sidebar. Having them in the sidebar wouldn't be bad, but I don't think it would be an improvement over the way I mocked it up. Those things could be tweaked afterhand, though. The most important issue right now is actually having a new tab page