I don't know if it would be for everyone, but for me yes. I have basically stopped bookmarking, because I end up with thousands and never find what I was looking for. Search the bookmarks is not enough. What would be cool is if a site could spider the pages of your own person bookmarks, so that you could search within the page of the bookmark. Not the whole site, just the one page. Or possibly if you bookmarked the index url, the whole site.
I know Google has custom search engines you can create, but that is not the same. I want to bookmark on the fly and then when I know I bookmarked something, search for it. That way I don't have to keep a catalog in my head of what site has what.
The site itself doesn't really have to do the bookmarking. It could leverage online bookmarking sites with their api's. In fact, that would probably have more potential, since people are already set in their ways about which bookmarking sites they like.
Why did I think of this? Because I have been sitting on tagreg8.com for a while, unsure of what to use it for. Do I have time to write it? No. So if any of you out there would like to play with the idea, let me know.