Typically a feed (or any list of items) will have links on each item to view them or perform actions on them. This means that users may well be navigating back and forth to these lists.

Imagine a list of news items; it's likely that a user would read the list, then choose one or more news items to read, each time navigating back to the list view. Don't simply reload the feed and put the user back to the start again, you monster!

Twitter shows the user how many "tweets" behind they are, allowing them to manually reload if they wish, but not altering the feed without their explicit action:

Twitter getting something right for a change

Of course, technically, the feed may well have changed in the time it took the user to read the story, butg, it's disorienting and difficult to use. Yes, this means additionally keeping track of where your user's scroll position is, but it's worth it for the usability benefit.