First and foremost is speed. With the newsgroup I could see in less than a second with a glance at a small area which groups had new postings and how many of each, without even making the newsreader active. If I am using the example Marc posted, I have to switch focus and mouse to scroll down to see everything.
Second reason is purely real-estate. With the old newsgroup, I had Thunderbird docked on the side of my second monitor at about 600px to 700px wide. This gave me room an entire screen for just Rhino, and properties, layers, named views / cplanes, material editor, lights on the other and still have room for the newsgroup (and my mail) passively sitting there.
Perhaps I’m just curmudgeonie, but I liked that the newsgroup was clean, with a lot of information clearly displayed in a small space. It let me have it displayed at all times without having to interact with it to get a general sense of what was going on. With this site, I have to pull up my browser, almost always make it bigger, then navigate to see what is new, instead of just looking at a glance. The “whatever, people have all the screen space in the world, we don’t need to streamline this puppy” train of thought is irksome to me.
It would also be nice if it were functional in a docked Rhino web browser tab, which it is very much not so.