This is presumably going to become more relevant, and have a bigger backlog over time as things change, but what's prompted this thought right now is Google's recent closure and total shut-down of Google Reader.
Now that there is absolutely no way to use either the Google Reader website, the Android Google Reader app, or the Google Reader API an awful lot of the questions in google-reader just aren't relevant or useful any more. While some of them are really more generic RSS related questions that happen to have the Google Reader tag, many of the others are very specific question about the app or service. While I don't think that these necessarily need deleting (there are answers on there that were previously good and useful, and they don't deserve to lose the associated rep just because things have changed), there's also no reason to leave the question open when it's never going to receive another good answer.
Edit: should probably also point out that Google Reader is just a topical example. In the future this could also apply to many other commonly used services that rely on a backend provided by a single company, eg Facebook, GMail, Twitter, Skydrive, Flickr, Dropbox and so on.