Nice analogy but like all analogies, it's too simplistic and flawed. You left out one important and critical part: the hypothetical passenger in your example is tied to a specific plane/airline. If you don't like your pilot or plane type, you cannot move to a different airline or request a different plane or a different pilot since you're chained to the specific plane.
Due to Google App Engine's API lock-in, you're stuck with them as a provider... quite possibly forever due to heavy BigTable dependency.
Even though I'm a huge fan of cloud computing, I'd rather use a strategy that uses platforms/planes that are built from reusable parts and allow you to switch your plane/airline provider as you please. Don't like Delta? Just go to AA counter and you don't have to change your luggage, clothing etc.
Until there's a second, GAE-compatible, ISV provider that offers full compatibility with GAE, I'd avoid GAE like a plague.