That's just the thing this is addressing: the fact that a lot of people end up using tabs just like bookmarks, because bookmarking is more of an archiving activity with no capability of saying "this is what's currently on my radar." There needs to be a happy medium available for things that are still "open" but not "actively being looked at this second".
I myself experience this conundrum all the time, but the difference is that if i run out of time/desire to look at all of my open tabs, i neither leave them open nor properly bookmark them, but rather drag them into my Bookmarks Toolbar, which is the closest thing to the tab bar but of course doesn't bog down the performance :). Needless to say, my solution is also quite disorganized and unsatisfactory.
I think you make a quite valid point however that there is a lot of overlap between this new interface and good old bookmarks. In fact, once this thing matures sufficiently, I wouldn't be surprised if it negates the need for bookmarks entirely for many users. However, it is undoubtedly better to let the users ease into this new paradigm and then realize that bookmarks are no longer needed, than to force them into it by removing bookmarks from the browser at this early stage.