I can already hear the complaints

"Why not just make it live action instead of making animation look photorealistic? The reason for making it look as photorealistic as possible was that it makes it seem much more like magic if a pair of umbrellas comes to life in a realistic world than in an more stylized and traditional animated world.

As the director himself says:

"We initially thought about shooting live action," he explains, "but if you start to go down that road in your head, shutting down a downtown city block with dozens of cars and extras and people with umbrellas and making it rain, and then trying to control that, you don't want to do that. We built all the shots in the computer and then in editorial we re-recorded shots with a separate camera capture unit for a hand-held look. This gave the impression of moving through physical space and a sense of weight."

Instead of creating the right light, mood, atmosphere and sets in the real world, or reshooting the scenes and be restricted to a few angles, making it in the computer also makes you much more free to where you want the camera to be, where to move and gives you other kinds of freedom you wouldn't have in live action.

And just as important; it's a challenge, something Pixar has never tried to do before. This gives them the oppertunity to solve new problems and push the limits in new directions, to learn and try out the kind of experiments that's not possible in feature movies. Hopefully it will also give inspiration to other filmmakers and animators.

No matter how the story turns out, I'm very curious about how the world they have created will look. And in my opinion; if it looks good enough, I would rather see a Marvel superhero done this way, with photorealistic animation, that with real life actors or performance capture.