100% agree. I had that question included in my first draft of my original comment, but figured anonymizing the bookmark data is a very tricky/impossible task so I removed it. See AOL search data[1]. Maybe Xmarks could sent an opt-in request to users and release the data of people who are comfortable with it. I would be fine with it, personally; all my bookmarks are on delicious anyway.
[1] http://en.wikipedia.org/wiki/AOL_search_data_scandal