I'll go out on a limb here and say that most apps handle some kind of data structure on the server, and in many cases they facilitate some kind of interaction b

I'll go out on a limb here and say that most apps handle some kind of data structure on the server, and in many cases they facilitate some kind of interaction between the user and this data. While you can theoretically do the entire functionality in client-side JavaScript, I assert the idea is so abysmally bad it's not really worth considering as a valid counter example. Please note that I wouldn't include pure single user applications like http://www.diagram.ly/ http://www.diagram.ly/ in this category because they don't manipulate objects on the server that are in need of protection and validation.