I think another use case would be where I access a blog article from an RSS feed, and the link contains something like www.foo.com/blog/post/1?rss=1; which I'll accidentally bookmark. What I really wanted to bookmark was www.foo.com/blog/post/1.
My advice for the development team would be to allow users to change the URL, but drop annotations/highlights for the page. Alert users with a pop-up, "You are changing the URL for this entry. YOU WILL LOSE ALL SAVED HIGHLIGHTS AND STICKY NOTES! Are you sure you wish to proceed? [Change URL] [Cancel]"
I find the limit of 10 tags per bookmark arbitrarily restrictive; the limit creates a disadvantage to an otherwise superior social bookmarking service. Please consider removing this limit or placing it at some significantly higher number (e.g. 30 tags). Diigo prides itself on doing more; tagging should not be an exception.
Briefly, you want tags to be associable in order to connect related concepts, but tag distribution has a "long tail". Limiting the tags cuts off this tail, severing many connections between concepts.
My advice for the development team would be to allow users to change the URL, but drop annotations/highlights for the page. Alert users with a pop-up, "You are changing the URL for this entry. YOU WILL LOSE ALL SAVED HIGHLIGHTS AND STICKY NOTES! Are you sure you wish to proceed? [Change URL] [Cancel]"