Debian also has a roughly 2 year release frequency. 2005, 2007, 2009, 2011. Stable is already frozen so 2013 is likely.

I thought Ubuntu's original reason for existing was because Debian releases don't happen often enough?

It existed because testing wasn't stable enough and stable wasn't fast enough. The 2 year LTS releases are basically just the slow stable thing carried over, but having 2 layers of rolling release (like Arch) where you have a base repo and a testing repo lets them having the rolling release without all the continuous integration problems if it hits enough testing users first.