> Almost all cats look the same

Congratulations, you've just dramatically simplified the algorithm. This means that once you can identify a cat, you can say that any image which isn't a cat is most likely a dog.

Also, cheer up! This isn't supposed to be "useful". Who cares if it's narrow and can't be applied to anything else? It's a chance to have a bit of fun and for some people (like me) it's a chance to learn about image recognition techniques.