This puzzled me as well, at first. It can be tad more annoying to see the animation play in the editor, but overall this change is for the best.
One thing that really frustrated me in v3 was that AnimatedSprite would constantly be changing it's frame in the editor. Then when you go to commit your code, the scene changed just by looking at it, but only the frame of the sprite... grr... this new behavior solves that