In my estimation, the ability to aggregate is one of the most important strengths of the whole RSS / Atom feed model. Of course, data push rather than pull, notification without polling (checking), is the keystone of what makes RSS different from the traditional web browsing user experience. That idea is captured neatly by the word feed. However, the alternate moniker usually involves some variation or abbreviation of the word syndication. The concept of syndication is usually accompanied by repackaging the disparate syndicated sources into a single, new, coherent whole.
Indeed, RSS readers are often called feed aggregators or news aggregators, but aggregation in its fullest sense is often not a key feature of these applications. The user interface for most RSS readers involves a listing of all subscribed feeds in one pane, and then the list of actual article data for the currently selected feed in a second pane. Sometimes the second pane contains a list of headlines from the selected feed, while a third pane contains the article data for the single selected headline. So yes, this is aggregation in the sense that the many feeds are being organized by the single RSS reader application, but the user must still choose which feed to read. The headlines and articles displayed are only drawn from the currently selected feed. This presents the user with an unnecessary choice, which makes reading all feeds more work than it should be. It also removes the benefit of directly comparing the chronological ordering of articles from disparate feeds.
The ability to restrict viewing to a single feed or group of feeds is surely beneficial, but the primary use case should be that of all feeds being aggregated into a single feed. It seems the most frequent use of an RSS reader is to create a personalized news source—an aggregation of the news feeds an individual finds interesting. The easiest and most logical way to read this is as a chronological listing of all articles.
In short, RSS is not email. As long as RSS clients insist on using interface metaphors borrowed from email clients, there is no compelling reason to use RSS (trendiness is not a real reason).
