You see, my typical usage scenario for RSS feeds is that I go through all of the new items and double-click those that I want to read more about. But I don’t read them right away. Once I’ve made it through the whole list, I mark the rest as read then switch to my browser to go through the entries I found interesting.
NetNewsWire has a preference (Browsing → Behavior) that let’s me open links in the background (i.e., without switching focus to my web browser). With Apple Mail, every time I clicked a “Read More…” link I was thrown into my browsers “space” and had to CMD-TAB back to Mail to continue looking for interesting things to read. Combine that with the fact that I had to find the “Read More…” link in the first place and I just couldn’t take it any more.