>People who have too many tabs open are almost always using their tabs as bookmarks, or as an "I want to read this later" mechanism
We can reverse the logic in your statement and it makes just as much sense. "People who have too many bookmarks are almost always using their bookmarks as tabs, or as an "I want to read this later" mechanism".
As a self proclaimed "tabs as a read it later" user, I do so because I've never found a better "read it later" mechanism. Maybe bookmarks work for you, but I find them mostly useless. For me, tabs are mostly self organizing with similar topics in the same window, and similar windows are close together on the taskbar. And I can reorganize them without opening some other UI to do it. And, the main benefit is, closing a tab removes it from my "read it later" queue. With a bookmark you have to go find it in your bookmarks list by name, possibly among other similarly named, mostly unorganized bookmarks and do a couple more mouse clicks and movements to delete it.
Before tabs, I'm sure all of us tabbers used bookmarks as a "read it later" mechanism. And although I probably do have some 6+ month old tabs, my del.icio.us queue of "toread" items spanned years. And that's a conservative estimate because I haven't used it in years and there's still articles on the list I haven't touched.
I absolutely welcome alternatives, but bookmarks isn't the answer for me.