I don't know if this anyone else remembers, but this isn't the first time Sakura decided to make herself more useful. The whole micro-arc in the Chunnin exam that resulted in her cutting off her hair was a pretty big jump for her, despite how little it actually changed her influence on the story (tied/lost to Ino, got captured by Gaara, cried a lot when Sasuke left).

Comparatively, this one is 100 times as huge. Medic-nin is perfect for the traits she has introduced at the beginning of the series. And medic-nin's are clearly really useful, without them, we would have lost probably 3-4 characters in the last arc.

Sakura's a main character, its about time she got a little more character development.