I guess it comes down to use-case. Would you remove the back/forward buttons in private mode?
If your workflow is "new tab for everything", then in essence opening a tab is like clicking a link, and closing the tab is like clicking "back". If the data is held in RAM only, and not persisted, then you aren't likely to be significantly impacting the threat model for the user. If you weren't allowing back/forward in private mode, I think users would dislike it, but then you could illustrate a scenario where there's a new security concern introduced.
From the perspective of "container tabs" that we've designed/talked about before, making "private browsing" usable will be important, as that enables the "disposable temporary container" in a more elegant way.