[jpalomaki]

From the comments in the article: " It certainly works much better as a broadcast medium and for giving occasional shout-outs, high fives, and anything you know isn't going to go beyond a short, positive interaction "

Looking at the Twitter design, this seems to be the intended use. At least in my stream almost all tweets are accompanied by a large picture. Quite often something that is not even very relevant, since they are auto picking the pictures for linked web sites. This makes very poor use of screen real estate. If conversations were a thing for them, I would expect a different design.