How Zynga Survived FarmVille

He mentions that FarmVille maintains both their own servers and EC2 servers. That's an interesting approach to maintain some baseline capacity locally and use the cloud for scaling. I suppose its obvious, but usually with the cloud I hear about all or nothing (well or just some specific task in the cloud).

The story is interesting, but I still can't figure out why having both their own servers and EC2 splitting 50/50 is a good idea. If EC2 dies the private server won't be able to handle all the traffic and FarmVille will stop working. If the private servers die, they will have a little hiccup until they manage to rescale EC2 and everything will be back to normal. With this system, in practice they already fully depend on EC2 and have an extra point of potential failure. What am I missing?