Google Reader (FMM rest its soul) had a nifty usability feature: when all feed items were read, it displayed "View all items" instead of items themselves (just like BazQux). However, it displayed all items only once, not switched feed to "Show all" mode.

Use case in point: I've accidentally scrolled too far, marking everything as read. Or I want to look up some of the recent items to share the link. Now I click button, which changes feed settings and disappears altogether, so I have to remember to change settings back, and do it in another place.