In addition to the others mentioned, regulatory issues come to mind. The company I work for hosts applications we are not allowed to host on servers we do not control.

To quote the question "Why would anyone want it on other cloud providers"

You can't use AppEngine or any other cloud provider, unless you start playing semantic games around what "servers we do not control" means.

Well, I'm happy with app engine so far, but it would be nice to have an insurance plan in case Google decide to do something which changes that. Possibilities include retiring app engine, changing the pricing significantly, introducing unpalatable restrictions, etc.

Because if they increase their prices, as they have done in the past already, you may still want the freedom of hosting your app somewhere else.

Misleading statement. They did not "increase" the price so to speak. The platform came out of beta and into normal pricing mode. The low prices were introductory prices to get developers to use the platform.

The price is higher than it was before, correct? In that case, it was increased.

Increased back to the original price. Yes. It was not higher than the original price and the prices have gone down a little since. Nice play on words there. :)

You are playing with words. The price of using the service increased dramatically.

even if they hadn't changed their prices to reflect their beta status, it doesn't mean their prices won't change to one which you no longer like. Having an open source alternative means you are not locked into the vendor, and they cannot charge you an exorbitant price.

appscale?

Also known as Bait and Switch.

For me, maintenance of large DBs can be very expensive. Some simple query like select count(*) from where ... can easily cost $10, and mysql will give you it for free.

vendor lock-in, risk management, privacy of data

For example, if your employer is a non-US government, and feels that feeding all your data directly to the NSA might be undesirable.

We host MIT App Inventor on Google App Engine. However we have some folks who want to use App Inventor in locations that have difficulty reaching GAE. Either because they are located on a poor internet connection, or don't have one at all!

I agree. The private case may be more appealing though. App developers might have some enterprise customers who want to run their applications behind a firewall.