Twitter Scroller scrolls so that the least-recent new message sits flush with the bottom edge of the viewport, or to the top of the page when that is not possible.
The default behaviour when you click the "X new tweet(s)" button is to not scroll at all. This probably makes more sense for most users, as it does not change perspective without warning.
That said, it is a pain, as the visual indication of where the new group of messages starts is very subtle (a slightly-darker separator line), and can be difficult to spot when more than a handful of new messages are loaded.
In other words, the Twitter Scroller extension helps you quickly see the new messages and get back quickly to your work, without wasting any time.
