I for one would find it useful. "This sounds interesting, but I'm a bit busy at the moment and worried that I may not come across this link again later."
You can do this by either hitting the favorite link or simply upvoting. History of both is retained in your profile (difference being that favorites are publicly visible).
Click "favorite"<p>Then you can see them at <a href="https://news.ycombinator.com/favorites?id=hank808">https://news.ycombinator.com/favorites?id=hank808</a>
Nope. This is not the job of the website.<p>There are plenty of other ways to more efficiently save bookmarks.<p>Also, one of the most advantages of HN in particular is its simplicity and minimalism.