Think of the poor animators! Personally, I think rotoscoping takes away from a scene unless it's meant to look outlandish. Ghost Trick on DS has some really crazy rotoscoped stuff, but the alienness of it adds to its charm which probably wouldn't be the case for most uses of it in anime. The /qa/ voice's dance there in OP looks very "unnatural" too, but it's pretty much meant to look that way.
Also, whoa, I didn't know webp can be animated.