Do you never find a page and think, "ah, this will be useful for project X later"?
Keeping them all in my browser session for six months before I put it to good use is just unnecessary, so I bookmark it and find it later when I need it.
Ah, now, you're on HN. This is the place where, in threads about browsers, we inevitably attempt to out-do each other with number of open tabs.
And sooner or later the guy shows up who hasn't closed his browser window or any tab in it since 2003, and now has essentially the entire Wayback Machine in memory.
I do, but most of the times it's been via Twitter, so I just fave it there. Or it's here, in which case I usually don't bookmark it.
Like Cthulhu_, I've been bookmarking for years till my bookmarks were filled and literally unusable; then I moved to Google Reader and carefully chosen RSS Feeds, which slowly became a bloated mess (more than 3,000 favorites there); then to Kippt, and bis repetita.
In the end, I've come to the conclusion that keeping bookmarks for "interesting stuff" isn't so interesting (no pun intended.) If it's a great thing and I must keep it in mind, it'll surely come back to the surface somehow.
I must add that I have an extremely complex memory: it is very small and forgetful when it comes to actual stuff I have to remember (like my friends's names or university lessons), and abnormally large when it comes to seemingly insignificant stuff (like ZURB's Foundation framework which is competing with Twitter's Boostrap). I'll usually remember all the URLs I've typed. I'm a strange weirdo.
Finally, I realize now that I'm quite the exception here in terms of bookmarks, but seeing the rise and fall of Delicious, as well as Xmarks's (one of the insignificant things I remember, even though I've never used the service or heard of it before it closed down) I thought it was more common.
Keeping them all in my browser session for six months before I put it to good use is just unnecessary, so I bookmark it and find it later when I need it.