What's the benefit here? App Engine already has a downloadable dev environment that gives you everything you need and is instantly updating as well. You can use the App Engine Launcher [0] to test multiple sites locally and then deploy to GAE. Best of all, you can operate at local speeds using an editor you're already comfortable with and it works even if you're sitting in a lead bunker with no connectivity.

What am I missing?

[0] http://googleappengine.blogspot.com/2008/05/app-engine-launc...