I have been using Google Reader as a historical record of posts as well as a way of keeping up to date with news. Among other things, it's enabled me to search for past articles in my feed that I may not necessarily have starred. (This has also been very helpful on occasions when a blog or site has closed but I've still wanted to refer to the articles it contained.)

But for this to work, all the past posts need to be imported from Google Reader. And, correspondingly, the service should keep all stories (not just the most recent ones).